I wonder how much of this was related to Lewis’ own education.  He did not enjoy public schooling.  His time at Wynyard School involved being terrorised by the school’s violent headmaster.  He also did not enjoy his time at Malvern College either.  He was much happier receiving personal tutelage from William Kirkpatrick.  In the Chronicles of Narnia, Private tutors are often represented favourably:

• Caspian receives positive education from Dr Cornelius, his private tutor,
• Peter receives private tuition from old Professor Kirke (whose name is remarkably similar to that of W. T. Kirkpatrick) during the events of ‘The Voyage of the Dawn Treader’.

Schools do not fare so well:

• Edmund is initially corrupted by his school.  Peter comments

…You’ve always liked being beastly to anyone smaller than yourself; we’ve seen that at school before now.

Later Lewis comments that Edmund becomes a more honest person than he had been in ages - “ever since his first term at that horrid school which was where he had begun to go wrong.”
(Again, many people notice that Lewis lost his faith as a child while attending what he considered to be a ‘horrid’ school.)

• As you pointed out the professor constantly wonders “what they teach kids at these schools”.
• Experiment house is not presented as the best school ever made:

These people had the idea that boys and girls should be allowed to do what they liked. And unfortunately what ten or fifteen of the biggest boys and girls liked best was bullying the others. All sorts of things, horrid things, went on which at an ordinary school would have been found out and stopped in half a term; but at this school they weren’t.

Again, Lewis own time at Malvern College was not a positive experience. Lewis did not like the manner in which the elder students forced younger students to ‘fag’ for them (i.e., do chores like clean the older children’s boots).  Lewis wrote pretty harshly about this school in Surprised by Joy.

It could be that Lewis had very different reasons for the strong views he expressed about schooling in the books, but I do wonder if his own negative experience of the British schooling systems is a factor in what he wrote.

Posted by: @waggawerewolf27

But what are the main themes, anyway, in each of the Chronicles of Narnia?

C.S. Lewis said “The Voyage of the Dawn Treader” is about

…the spiritual life (specially in Reepicheep).

I find this statement both helpful and confusing.  The first half of this sentence makes perfect sense to me, as the characters have a number of episodes throughout the book (with a different character’s point-of-view presented in each episode).  In each episode, the character is often tempted / tested and grows through the experience – with Aslan usually delivering the person to some degree.  Thus, the theme of ‘the spiritual life’ sums up this book very well.

‘Especially Reepicheep’ on the other hand does not help me much at all.  Reepicheep is never the point-of-view, main character in any of these episodes.

Reepicheep has a conflict early in the story with Eustace.

"Ugh, take it away," wailed Eustace. "I hate mice. And I never could bear performing animals. They're silly and vulgar and — and senti mental."

"Am I to understand," said Reepicheep to Lucy after a long stare at Eustace, "that this singularly discourteous person is under your Majesty's protection? Because, if not — "

Anyway, as soon as he saw that long tail hanging down — and perhaps it was rather tempting — he thought it would be delightful to catch hold of it, swing Reepicheep round by it once or twice upside-down, then run away and laugh. At first the plan seemed to work beautifully. The Mouse was not much heavier than a very large cat. Eustace had him off the rail in a trice and very silly he looked (thought Eustace) with his little limbs all splayed out and his mouth open. But unfortunately Reepicheep, who had fought for his life many a time, never lost his head even for a moment. Nor his skill. It is not very easy to draw one's sword when one is swinging round in the air by one's tail, but he did. And the next thing Eustace knew was two agonising jabs in his hand which made him let go of the tail; and the next thing after that was that the Mouse had picked itself up again as if it were a ball bouncing off the deck, and there it was facing him, and a horrid long, bright, sharp thing like a skewer was waving to and fro within an inch of his stomach. (This doesn't count as below the belt for mice in Narnia because they can hardly be expected to reach higher.)

"Stop it," spluttered Eustace, "go away. Put that thing away. It's not safe. Stop it, I say. I'll tell Caspian. I'll have you muzzled and tied up."

"Why do you not draw your own sword, poltroon!" cheeped the Mouse. "Draw and fight or I'll beat you black and blue with the flat." "I haven't got one," said Eustace. "I'm a pacifist. I don't believe in fighting."

"Do I understand," said Reepicheep, withdrawing his sword for a moment and speaking very sternly, "that you do not intend to give me satisfaction?"

Despite Reepicheep’s dislike of Eustace, he refuses to abandon Eustace when Rinse implies that the heroes are better off without him:

"But we must do something," said Lucy. "He may have got lost, or fallen into a hole, or been captured by savages."
"Or killed by wild beasts," said Drinian.
"And a good riddance if he has, I say," muttered Rhince.
"Master Rhince," said Reepicheep, "you never spoke a word that became you less. The creature is no friend of mine but he is of the Queen's blood, and while he is one of our fellowship it concerns our honour to find him and to avenge him if he is dead."

Reepicheep shows compassion for Eustace when Eustace is a dragon.

On the evenings when he was not being used as a hot-water bottle he would slink away from the camp and lie curled up like a snake between the wood and the water. On such occasions, greatly to his surprise, Reepicheep was his most constant comforter. The noble Mouse would creep away from the merry circle at the camp-fire and sit down by the dragon's head, well to the windward to be out of the way of his smoky breath. There he would explain that what had happened to Eustace was a striking illustration of the turn of Fortune's wheel, and that if he had Eustace at his own house in Narnia (it was really a hole not a house and the dragon's head, let alone his body, would not have fitted in) he could show him more than a hundred examples of emperors, kings, dukes, knights, poets, lovers, astronomers, philosophers, and magicians, who had fallen from prosperity into the most distressing circumstances, and of whom many had recovered and lived happily ever afterwards. It did not, perhaps, seem so very comforting at the time, but it was kindly meant and Eustace never forgot it.

Upon encountering the Sea Serpent, Reepicheep is the one who points out that it is no use attacking the Sea Serpent, but rather the whole crew needs to work together to push the serpent away:

Others would have joined him if at that moment Reepicheep had not called out, "Don't fight! Push!"

…

The brute had made a loop of itself round the Dawn Treader and was beginning to draw the loop tight. When it got quite tight — snap! — there would be floating matchwood where the ship had been and it could pick them out of the water one by one. Their only chance was to push the loop backward till it slid over the stern; or else (to put the same thing another way) to push the ship forward out of the loop.

Reepicheep alone had, of course, no more chance of doing this than of lifting up a cathedral, but he had nearly killed himself with trying before others shoved him aside. Very soon the whole ship's company except Lucy and the Mouse (which was fainting) was in two long lines along the two bulwarks, each man's chest to the back of the man in front, so that the weight of the whole line was in the last man, pushing for their lives.

Reepicheep is again the voice of reason (and courage) on the island of the Duffers.  He repeatedly informs the company that they cannot avoid the fate the invisible Duffers force upon them and calls them to bravely face it:

"Your Majesties all," said Reepicheep, "hear me. It is folly to think of avoiding an invisible enemy by any amount of creeping and skulking. If these creatures mean to bring us to battle, be sure they will succeed. And whatever comes of it I'd sooner meet them face to face than be caught by the tail."

"Her Majesty is in the right," said Reepicheep. "If we had any assurance of saving her by battle, our duty would be very plain. It appears to me that we have none. And the service they ask of her is in no way contrary to her Majesty's honour, but a noble and heroical act. If the Queen's heart moves her to risk the magician, I will not speak against it."

It is Reepicheep who succeeds where Lucy could not of convincing the Dufflepuds that being a monopod really isn’t that bad. He does this by being empathetic and helping them to focus on what is good in their lot:

When they reached the bay, Reepicheep had a brilliant idea. He had his little coracle lowered and paddled himself about in it till the Monopods were thoroughly interested. He then stood up in it and said, "Worthy and intelligent Monopods, you do not need boats. Each of you has a foot that will do instead. Just jump as lightly as you can on the water and see what happens."
The Chief Monopod hung back and warned the others that they'd find the water powerful wet, but one or two of the younger ones tried it almost at once; and then a few others followed their example, and at last the whole lot did the same. It worked perfectly. The huge single foot of a Monopod acted as a natural raft or boat, and when Reepicheep had taught them how to cut rude paddles for themselves, they all paddled about the bay and round the Dawn Treader, looking for all the world like a fleet of little canoes with a fat dwarf standing up in the extreme stern of each. And they had races, and bottles of wine were lowered down to them from the ship as prizes, and the sailors stood leaning over the ship's sides and laughed till their own sides ached.

If not for Reepicheep, the Dawn Treader’s crew would not have sailed into the Dark Island and found the Lord Rhoop (neither would they have been endangered by the nightmares of this island):

… But all at once the clear voice of Reepicheep broke in upon the silence.
"And why not?" he said. "Will someone explain to me why not."
No one was anxious to explain, so Reepicheep continued:
"If I were addressing peasants or slaves," he said, "I might suppose that this suggestion proceeded from cowardice. But I hope it will never be told in Narnia that a company of noble and royal persons in the flower of their age turned tail because they were afraid of the dark."
"But what manner of use would it be ploughing through that black ness?" asked Drinian.
"Use?" replied Reepicheep. "Use, Captain? If by use you mean filling our bellies or our purses, I confess it will be no use at all. So far as I know we did not set sail to look for things useful but to seek honour and adventures. And here is as great an adventure as ever I heard of, and here, if we turn back, no little impeachment of all our honours."

Reepicheep is the one who will fulfill the quest’s requirement of going as far as he can to the world’s end.  Reepicheep knows that he may well be going to his death, but he desires it nonetheless (as he believes it is his life’s calling:

"Sir," said Caspian, "will you tell us how to undo the enchantment which holds these three Narnian Lords asleep."
"I will gladly tell you that, my son," said the Old Man. "To break this enchantment you must sail to the World's End, or as near as you can come to it, and you must come back having left at least one of your company behind."
"And what is to happen to that one?" asked Reepicheep.
"He must go on into the utter east and never return into the world."
"That is my heart's desire," said Reepicheep.

And Reepicheep fulfils his quest – perhaps with fear, but nonetheless boldly and with great courage.  As one person on the internet put it, “He ends his journey joyfully”:

No one in that boat doubted that they were seeing beyond the End of the World into Aslan's country.
…
"This," said Reepicheep, "is where I go on alone."
They did not even try to stop him, for everything now felt as if it had been fated or had happened before. They helped him to lower his little coracle. Then he took off his sword ("I shall need it no more," he said) and flung it far away across the lilied sea. Where it fell it stood upright with the hilt above the surface. Then he bade them good-bye, trying to be sad for their sakes; but he was quivering with happiness. Lucy, for the first and last time, did what she had always wanted to do, taking him in her arms and caressing him. Then hastily he got into his coracle and took his paddle, and the current caught it and away he went, very black against the lilies. But no lilies grew on the wave; it was a smooth green slope. The coracle went more and more quickly, and beautifully it rushed up the wave's side. For one split second they saw its shape and Reepicheep's on the very top. Then it vanished, and since that moment no one can truly claim to have seen Reepicheep the Mouse. But my belief is that he came safe to Aslan's country and is alive there to this day.

Thus, Reepicheep is indeed characterised as one living ‘the spiritual life’.  But, for myself at least, it is striking that Reepicheep is never the main character in any of the Dawn Treader’s episodes.  The book’s introduction is mostly told from Lucy’s perspective (until Eustace’s diary takes over).  The events on the Lone Islands are mostly told from Caspian’s perspective.  The events of the storm and Dragon Island are largely told from Eustace’s perspective.  On the island of the Duffers, Lucy’s perspective is key.  During the dark island, several perspectives are shown, though Caspian’s, Edmund’s and Lucy’s are the audience’s main views into the scene.  And in the voyage past Ramandu’s Island, it is again Lucy’s perspective that dominates the story.  Reepicheep is always a secondary character in each narrative.  Maybe, this just shows that I completely miss Lewis’s point in this story.  I am far more impressed in how the other characters engage with temptation and how Aslan helps them to overcome it as revealing the nature of ‘the Spiritual life’ in this story.

Kennedy Unthank thinks that Aslan is depicting the Holy Spirit in this book; convicting and comforting and protecting them from falling into sin.  He also sees Eustace’s subplot as demonstrating justification and sanctification:

King Caspian, Edmund and Lucy all face temptations that nearly cause them to forsake each other. Caspian and Edmund nearly come to blows on an island that turns things to gold, and Lucy nearly casts a dangerous but alluring spell. However, in each instance, Aslan appears, protecting them from falling into sin. And when their ship becomes lost in a land of nightmares, Lucy calls on Aslan to help them, and he leads them out of the darkness and into the daylight.

Dr. Don W. King saw this book as related to greed.  Although the only example he relates this theme to is Eustace and Dragon Island.  I thought he could easily include Deathwater Island in this same assessment.

Similarly, in Planet Narnia, Dr Michael Ward presents a surprisingly similar thesis to that of Dr King (though this time with more detail).  Michael Ward sees this as the “Sunna (sun) / Sol / Apollo / Helios” story in the Narniad.  I think this is the strongest of his claims as once you remove the ‘sun’ related elements from this story, there is not a whole lot left over:

The voyage constitutes of travelling eastward.  The journey east naturally involves travelling towards the rising sun (treading the way to the dawn – as the name “Dawn Treader” implies):

"Right. Well, on my coronation day, with Aslan's approval, I swore an oath that, if once I established peace in Narnia, I would sail east myself for a year and a day to find my father's friends or to learn of their deaths and avenge them if I could ...

"…Why should we not come to the very eastern end of the world? And what might we find there? I expect to find Aslan's own country. It is always from the east, across the sea, that the great Lion comes to us."

And now the winds which had so long been from the north west began to blow from the west itself and every morning when the sun rose out of the sea the curved prow of the Dawn Treader stood up right across the middle of the sun.

Gold (the metallurgy of the sun) shows up regularly in this book:

• We are introduced to Caspian as a “Golden-headed boy some years older than herself .”
• Reepicheep’s gold ring that he wears on his head appears only in this book.
• Eustace is turned into a Dragon while sleeping on the golden coins of a Dragon’s hoard and while wearing a golden bracelet on his arm.
• Deathwater Island has a pool that turns everything that touches it (the Lord, the spear, the toes of Edmund’s boots, the heather Caspian dips in the water) into gold.
• Aslan is often depicted as golden (though he is in all of the Narnian chronicles, so perhaps that’s not too surprising:

But when she looked back at the opening words of the spell, there in the middle of the writing, where she felt quite sure there had been no picture before, she found the great face of a lion, of The Lion, Aslan himself, staring into hers. It was painted such a bright gold that it seemed to be coming towards her out of the page; and indeed she never was quite sure afterwards that it hadn't really moved a little.

Aslan has spoken to me. No — I don't mean he was actually here. He wouldn't fit into the cabin, for one thing. But that gold lion's head on the wall came to life and spoke to me.

Apollos was the lizard/Dragon slayer and Dragons show up a lot in this book:

• The ship, ‘The Dawn Treader’ is sculpted with a dragon’s head, wings and tail.
• On Dragon Island, Eustace encounters a Dragon & Eustace becomes a Dragon, himself.
• The Dawn Treader encounters a Sea serpent (which has an overlap with mythological sea dragons – though they are not identical – so this could be coincidental).

Light, brightness and the sun itself show up all over the place in this book:

• During the storm, it is very dark: “they never saw the sun.”
• During the drought following the storm, Eustace notes “Pretty hot sun.”
• The sun gets bigger as they sail eastward:

“Some thought that the sun looked larger than it looked from Narnia, but others disagreed.”
“Once or twice before, the Narnians had wondered whether the sun at its rising did not look bigger in these seas than it had looked at home. This time they were certain. There was no mistaking it. And the brightness of its ray on the dew and on the table was far beyond any morning brightness they had ever seen.”

...
“The sun when it came up each morning looked twice, if not three times, its usual size.”
“As I have said before, there had been too much light ever since they left the island of Ramandu — the sun too large (though not too hot), the sea too bright, the air too shining.”

• The Darkness of the Dark Island is opposed to light:

About nine that morning, very suddenly, it was so close that they could see that it was not land at all, nor even, in an ordinary sense, a mist. It was a Darkness. It is rather hard to describe, but you will see what it was like if you imagine yourself looking into the mouth of a railway tunnel — a tunnel either so long or so twisty that you cannot see the light at the far end. And you know what it would be like. For a few feet you would see the rails and sleepers and gravel in broad daylight; then there would come a place where they were in twilight; and then, pretty suddenly, but of course without a sharp dividing line, they would vanish altogether into smooth, solid blackness. It was just so here.
”
“Lucy, up in the fighting-top, had a wonderful view of the exact moment at which they entered the darkness. The bows had already disappeared before the sunlight had left the stern. She saw it go. At one minute the gilded stern, the blue sea, and the sky, were all in broad daylight: next minute the sea and sky had vanished, the stern lantern — which had been hardly noticeable before — was the only thing to show where the ship ended.”
“In a few moments the darkness turned into a greyness ahead, and then, almost before they dared to begin hoping, they had shot out into the sunlight and were in the warm, blue world again.”

• Ramandu is made younger by the fire-berries from the sun:
  “I am not so old now as I was then. Every morning a bird brings me a fire-berry from the valleys in the Sun, and each fire-berry takes away a little of my age.”
• Lucy notices that she is seeing the underwater world by way of the shadow cast by the sun:

Now Lucy knew she had seen something just like that happen somewhere else — if only she could remember where. She held her hand to her head and screwed up her face and put out her tongue in the effort to remember. At last she did. Of course! It was like what you saw from a train on a bright sunny day. You saw the black shadow of your own coach running along the fields at the same pace as the train. Then you went into a cutting; and immediately the same shadow f licked close up to you and got big, racing along the grass of the cutting-bank. Then you came out of the cutting and — flick! — once more the black shadow had gone back to its normal size and was running along the fields.
"It's our shadow! — the shadow of the Dawn Treader" said Lucy. "Our shadow running along on the bottom of the sea. …

• The light of the sun grows brighter, but the voyagers abroad the Dawn Treader are able to look at the brightness around them:

“Now, the light grew no less — if anything, it increased — but they could bear it. They could look straight up at the sun without blinking. They could see more light than they had ever seen before. And the deck and the sail and their own faces and bodies became brighter and brighter and every rope shone. And next morning, when the sun rose, now five or six times its old size, they stared hard into it and could see the very feathers of the birds that came flying from it.”

...
“They could not have seen even the sun if their eyes had not been strengthened by the water of the Last Sea. But now they could look at the rising sun and see it clearly and see things beyond it. What they saw — east ward, beyond the sun — was a range of mountains.”

• The water of the sea at the world’s end is ‘drinkable light’:

"Yes," he said, "it is sweet. That's real water, that. I'm not sure that it isn't going to kill me. But it is the death I would have chosen — if I'd known about it till now."
"What do you mean?" asked Edmund.
"It — it's like light more than anything else," said Caspian.
"That is what it is," said Reepicheep. "Drinkable light. We must be very near the end of the world now."

• When the children leave Narnia, it is through a door that shows a bright light in the sky:

“…Come, I am opening the door in the sky." Then all in one moment there was a rending of the blue wall (like a curtain being torn) and a terrible white light from beyond the sky, and the feel of Aslan's mane and a Lion's kiss on their foreheads and then — the back bedroom in Aunt Alberta's home at Cambridge.

• Aslan appears to be a source of light when our heroes encounter him (he is ‘the light of the world’ in this story):

And one queer thing was that there was no moon last night, but there was moonlight where the lion was.
…
And it led me a long way into the mountains. And there was always this moonlight over and round the lion wherever we went.

Across the grey hillside above them — grey, for the heather was not yet in bloom — without noise, and without looking at them, and shining as if he were in bright sunlight though the sun had in fact gone in, passed with slow pace the hugest lion that human eyes have ever seen. In describing the scene Lucy said afterwards, "He was the size of an elephant," though at another time she only said, "The size of a cart-horse." But it was not the size that mattered. Nobody dared to ask what it was. They knew it was Aslan.

• Aslan appears as an Albatross in a beam of light:

"Look!" cried Rynelf's voice hoarsely from the bows. There was a tiny speck of light ahead, and while they watched a broad beam of light fell from it upon the ship. It did not alter the surrounding dark ness, but the whole ship was lit up as if by a searchlight. Caspian blinked, stared round, saw the faces of his companions all with wild, fixed expressions. Everyone was staring in the same direction: behind everyone lay his black, sharply-edged shadow.
Lucy looked along the beam and presently saw something in it. At first it looked like a cross, then it looked like an aeroplane, then it looked like a kite, and at last with a whirring of wings it was right overhead and was an albatross. It circled three times round the mast and then perched for an instant on the crest of the gilded dragon at the prow. It called out in a strong sweet voice what seemed to be words though no one understood them. After that it spread its wings, rose, and began to fly slowly ahead, bearing a little to starboard. Drinian steered after it not doubting that it offered good guidance. But no one except Lucy knew that as it circled the mast it had whispered to her, "Courage, dear heart", and the voice, she felt sure, was Aslan's, and with the voice a delicious smell breathed in her face.

• Aslan appears as a small lamb, too bright to look at even with the light adapted eyes the children have at the end of the book:

But between them and the foot of the sky there was something so white on the green grass that even with their eagles' eyes they could hardly look at it. They came on and saw that it was a Lamb.
"Come and have breakfast," said the Lamb in its sweet milky voice.
Then they noticed for the first time that there was a fire lit on the grass and fish roasting on it. They sat down and ate the fish, hungry now for the first time for many days. And it was the most delicious food they had ever tasted.
"Please, Lamb," said Lucy, "is this the way to Aslan's country?"
"Not for you," said the Lamb. "For you the door into Aslan's country is from your own world."
"What!" said Edmund. "Is there a way into Aslan's country from our world too?"
"There is a way into my country from all the worlds," said the Lamb; but as he spoke his snowy white flushed into tawny gold and his size changed and he was Aslan himself, towering above them and scattering light from his mane.

(in)Visibility is featured on the island of the Duffers (we see light – so this is still the same motif):

• Lucy makes the Dufflepuds and Coriarkin visible:

"When will the spell work?" asked Lucy. "Will the Duffers be visible again at once?"

"Oh yes, they're visible now. But they're probably all asleep still; they always take a rest in the middle of the day."

• Lucy makes Aslan visible in Coriarkin’s mansion:

"Oh, Aslan," said she, "it was kind of you to come." "I have been here all the time," said he, "but you have just made me visible." "Aslan!" said Lucy almost a little reproachfully. "Don't make fun of me. As if anything I could do would make you visible!" "It did," said Aslan. "Do you think I wouldn't obey my own rules?"

The sun is the only one of ‘the seven stars’ / ‘the seven planets’ / ‘seven heavens’ that is a star.  In this book, we meet anthropomorphic stars.

• Coriakin is a star who is punished into looking after the Duffers
• Ramandu is a star at rest
• Caspian marries the star’s daughter

Most importantly, from a thematic perspective, ‘Sol’ represents liberality (generosity and altruism) as opposed to greed and selfishness, Ward’s website says:

Sol's characteristic influence was to illuminate the human mind, making people wise and liberal.  'Liberal' here means generous, free, and opposed to cupidity and utilitarianism. Sol burns away base considerations of greed and profit.  He 'hurts and humbles', as Lewis put it in his poem, 'The Planets'.

In “The Discarded Image”, C.S. Lewis says:

makes men wise and liberal and his sphere is the Heaven of the theologians and philosophers. …”

And in “Studies on Words”, Lewis writes:

Liber is ‘free’, not a slave; or free, used of an inanimate object, in the sense of unconfined, unopposed.  The sea, in Ovid, as opposed to the rivers, is the plain of freer (liberioris) water.  One’s mind or judgements can be liber when one is not ‘committed’ or bound by previous engagement or prejudice.  Honest jurymen who come to the case with an ‘open’ mind are liberi solutique in Cicreo’s Verrines, ‘free without ties’.  Conduct is liberalis when it is such as becomes a freeman.  Justice, according to Cicero, is the most magnificent virtue and most suitable-to-a-freeman (liberalis).  This ethical sense is often specialised and narrowed to denote the quality we call liberality.  ‘Liberales are the sort of people who ransom prisoners of war’.

Ward sees this liberality mostly relating to King Caspian’s behaviour; he comments:

…  Caspian, that ‘golden-headed boy’ (a glance, perhaps at Apollo Chrysocomes, ‘Apollo of the golden locks’), is conspicuously generous: he provides ale for the old salts, rum for the ship’s company after the fight with the Sea Serpent, ‘grog all round’ following their escape from the Dark Island, and he promises ‘gold or land enough’ to make the sailors rich if they will accompany him to the utter East.  More significantly, he helps bring about the release of Pug’s slaves on the Lone Islands by offering a cask of wine to the slovenly guards at Narrowhaven, forgiving Gumpas his debt, and reimbursing both Lord Bern and the Calormene traders.  Thus generosity is put in the service of ‘freedom,’ for, as he writes in Studies on words, “Liberals are the sort of people who ransom prisoners,’ and he quotes Lord Berners (whose name presumably suggested that of Lord Bern): ‘He and all his companye shal depart frank and free at their pleasure.’

I would argue that we can see a similar liberality in Aslan’s freeing of Eustace from his enslavement as a dragon.  (Eustace’s dragoning was in turn caused, originally, by Eustace’s greedy, self-centred, illiberal behaviour when he forsook the ship’s company and was infatuated with the idea of taking the Dragon’s hoard for himself to live a life of comfort and luxury.)

At Deathwater Island, a sense of greed takes over both Caspian and Edmund, as they each want to be the caretaker of the gold:

"The King who owned this island," said Caspian slowly, and his face f lushed as he spoke, "would soon be the richest of all the Kings of the world. I claim this land for ever as a Narnian possession. It shall be called Goldwater Island. And I bind all you to secrecy. No one must know of this. Not even Drinian — on pain of death, do you hear?"
"Who are you talking to?" said Edmund. "I'm no subject of yours. If anything it's the other way round. I am one of the four ancient sovereigns of Narnia and you are under allegiance to the High King my brother."
"So it has come to that, King Edmund, has it?" said Caspian, laying his hand on his sword-hilt.

In Coriakin’s house, Lucy is greedy for the attention that Susan has been receiving for being ‘the beauty of the family’.  She does not care that wars will break out over her beauty in Narnia nor that no one will care about Susan compared with herself.  She is behaving self-centredly.

This links directly to the idea that Dr. Don W. King and @waggawerewolf27 present; namely that this book is related to the sin of greed and the virtue of liberality.  It seems there is a general consensus that this book relates to the sin of greed and the virtue of liberality.

Elsewhere, people seem to think the book combines several themes without any dominating.

One website lists the themes of this book as bravery (especially Reepicheep), Christianity (for Eustace being undragoned and the journey to Aslan’s country), Growing up + selflessness (Eustace on Dragon Island) and Temptation (most of the characters throughout the story).

Another site reports that it is about biblical repentance and conversion (Eustace’s undragoning), an anti-materialism world view (even in our world a star is more than a ball of flaming gas), pursuit of a better world (Reepicheep’s longing for Aslan’s country) and meeting Aslan in our world (the ending).

One user on reddit said simply that it is about the Christian life, in that Reepicheep pursues God’s Kingdom and the other characters struggle with temptation.  Which I thought was not a bad summary.

For myself, looking at ‘The Voyage of the Dawn Treader’ I notice:

During their time on The Lone Islands, the story is primarily told from Caspian’s point of view.  This is early in the book and there is little temptation yet.  We see the courage of Caspian and the Narnians as they conquer the Lone Islands cleverly with only 30 men.  This section is also anti-slavery (linking to liberality) as we see the tragedy of the enslavement of our heroes and the victory of Caspian conquering slavery.  This sequence presents our heroes in their best light; with the exception of Eustace, our heroes are all behaving heroically.

The narrative of the storm and Dragon Island follows Eustace.  Eustace is a self-indulgent, self-unaware, obnoxious, narrowminded boy.  After the storm, he justifies stealing water from the ship’s meagre rations.  Upon arriving on Dragon Island, he slips away to avoid doing his share of the work.  Once he sees the gold and treasure in the Dragon’s lair, he thinks only of how he can use this wealth to benefit himself.  Eustace is greedy, but he is also selfish in many other ways.  Upon turning into a dragon, Eustace realises just how selfish he has been.  Eustace character transforms due to his hardship.  The compassion of Reepicheep and the friendship he forms with Eustace in this story is beautiful.  Eustace becomes generous, simply enjoying ‘liking people and being liked’.  He also is keen to help, making fires, killing sheep and finding a new mast for the Dawn Treader.  In the midst of Eustace’s trial, it is only Aslan who is able to free him and make him a boy again.  The themes of temptation, redemption, greed and selfishness all show up in a big way in this section.

The encounter with the Sea Serpent also shows that while Eustace is not perfect, that he is genuinely on his way to becoming a better person.

On Deathwater Island, there is no one point-of-view character.  Caspian and Edmund both argue over who has the authority and the lust for wealth is also present (at least for Caspian).  Aslan again delivers our heroes from their temptation.  Greed and power both show up as themes in this section of the book.

On the Island of the Duffers, Lucy is the main character.  As she reads the magician’s book, Lucy is tempted by jealousy and vanity – desiring to be more beautiful than Susan.  She is also insecure, and vainly wants to know what her friends think of her.  Aslan is of course there to prevent her from uttering the spell to make herself beautiful beyond the lot of mortals and to comfort her after she eavesdrops on her friend.  Lucy’s courage in spite of her fear is also contrasted with the stupidity and cowardice of the duffers.  The primary themes in this section appear to me to be courage, vanity and temptation.

As the Dawn Treader travels through the darkness around the Dark Island, people begin to have hallucinations while they are near the island, in the darkness.  At the story’s climax, we get Lucy’s point of view as she calls out to Aslan for help and Aslan appears in the form of an Albatross to lead them out of and ultimately to dispel the darkness.  Fear and despair verses hope seems to be the big themes in this passage; the entire crew is tempted to despair of ever getting out of the darkness.  Aslan is their hope even in their hopeless situation.

The events at Ramandu’s Island has no clear main character.  Lewis includes some descriptions of what a given character is thinking, but he shares insights into all five of the main characters without favouring any one in particular.  No one is tempted in this narrative and Aslan does not need to deliver anyone.  There is some misinformation about how the lords were enchanted - with the characters fearing the food of Aslan’s table.  There is wrath and disagreement amongst the Telmarine lords in their backstory, but this is not a major element in this portion of the book.  The main theme I see here is simply Reepicheep’s desire to seek out his calling to the end of the world and Aslan’s country.

In the waters of the end of the world, there is again not really a main character.  Lucy’s viewpoint is the one the narrative usually takes up when there is one (e.g., watching the Mer-people).  Caspian vanity, pride and slothfulness in abandoning his throne are the main conflict in this section.  Aslan again confronts Caspian and helps him through his temptation.  Reepicheep’s enthusiasm and courage as he approaches the end of the world is also in focus, thematically, here.

Trying to summarize all of that in a word, I think I would say this book is primarily about temptation (or perhaps I would follow Lewis’ lead and say it is about the process of sanctification in daily life).  Assigning a sin to it is difficult.  I think your assignment, @waggawerewolf27, of greed and liberality works pretty well as these can be overarching themes to cover the others.

@waggawerewolf27, in your presentation of each of the seven Lords, you commented that Lord Bern represents ‘Sloth’.  The Narrative with the Lone Islands does seem to involve a ‘neglect of duty’ and apathy.  Perhaps I am being harsh in relating the initial capture to ‘sloth’, but our heroes are initially captured when they take some leisure on the lonely island of “Felimath”.  The crew is working hard, rowing the Dawn Treader to double the cape.  Caspian, Lucy, Edmund, Eustace and Reepicheap are not involved in this work, but instead go for a stroll on this island to meet the crew on the far side (which likely meant extra work for the other crew members).  I say this might be a harsh criticism as people do need to rest.  On the other hand, Lewis makes it clear that there is work to be done and our heroes do not take part in the work.  Instead, they create more work by touring a local island to little purpose:

"Are we to put in here, Sire?" asked Drinian.
"I shouldn't think it would be much good landing on Felimath," said Edmund. "It was almost uninhabited in our days and it looks as if it was the same still. The people lived mostly on Doorn and a little on Avra — that's the third one; you can't see it yet. They only kept sheep on Felimath."
"Then we'll have to double that cape, I suppose," said Drinian, "and land on Doorn. That'll mean rowing."
"I'm sorry we're not landing on Felimath," said Lucy. "I'd like to walk there again. It was so lonely — a nice kind of loneliness, and all grass and clover and soft sea air."
"I'd love to stretch my legs too," said Caspian. "I tell you what. Why shouldn't we go ashore in the boat and send it back, and then we could walk across Felimath and let the Dawn Treader pick us up on the other side?"
If Caspian had been as experienced then as he became later on in this voyage he would not have made this suggestion; but at the moment it seemed an excellent one. "Oh do let's," said Lucy.

Pug and the slavers show a ‘spiritual apathy’ and an indifference towards their responsibility to their fellow men.  The slavers show a complete apathy in regards to the morality of their trade:

"So that's what you are," said Caspian. "A kidnapper and slaver. I hope you're proud of it."
"Now, now, now, now," said the slaver. "Don't you start any jaw. The easier you take it, the pleasanter all round, see? I don't do this for fun. I've got my living to make same as anyone else."

As you pointed out, @waggawerewolf27, Lord Bern is somewhat derelict of his duties.  We are told at the start of the book:

… Perhaps you remember that when I was a child my usurping uncle Miraz got rid of seven friends of my father's (who might have taken my part) by sending them off to explore the unknown Eastern Seas beyond the Lone Islands."

The Lords were on a quest to discover the lands beyond the Lone Islands, but Lord Bern only got as far as the Lone Islands and decided to end his quest before he had even started it.  This may be an unfair view of Lord Bern as the quest he had committed himself too appears to have little purpose beyond curiosity.  His reasons of wanting to settle down, have a family and provide employment on the Lone Islands does not seem like an irresponsible lifestyle.  On the other hand, he abandons his quest with little struggle:

"My Lord Bern," said Caspian, "we must talk of the state of these Islands. But first what is your Lordship's own story?"
"Short enough, Sire," said Bern. "I came thus far with my six fellows, loved a girl of the islands, and felt I had had enough of the sea. And there was no purpose in returning to Narnia while your Majesty's uncle held the reins. So I married and have lived here ever since."

Lord Bern, once he is Duke of the Lone Islands does not want Caspian to fulfil his own quest (again with good reason).  Lord Bern still does not have much commitment to quests:

"I've often been up here of a morning," said the Duke, "and seen the sun come up out of the sea, and sometimes it looked as if it were only a couple of miles away. And I've wondered about my friends and wondered what there really is behind that horizon. Nothing, most likely, yet I am always half ashamed that I stayed behind. But I wish your Majesty wouldn't go. We may need your help here. This closing the slave market might make a new world; war with Calormen is what I foresee. My liege, think again."
"I have an oath, my lord Duke," said Caspian. "And anyway, what could I say to Reepicheep?"

The Lone Islands are run in a generally slothful manner:

In those days everything in the islands was done in a slovenly, slouching manner. Only the little postern opened and out came a tousled fellow with a dirty old hat on his head instead of a helmet, and a rusty old pike in his hand. He blinked at the flashing figures before him. "Cam — seez — fishansy," he mumbled (which was his way of saying, "You can't see his Sufficiency"). "No interviews without 'pointments 'cept 'tween nine 'n' ten p.m. second Saturday every month."

The Governor’s guards in the castle are slothful in their appearance and how they guard the courtyard:

Two of Caspian's men stepped through the postern and after some struggling with bars and bolts (for everything was rusty) flung both wings of the gate wide open. Then the king and his followers strode into the courtyard. Here a number of the governor's guards were lounging about and several more (they were mostly wiping their mouths) came tumbling out of various doorways. Though their armour was in a disgraceful condition, these were fellows who might have fought if they had been led or had known what was happening; so this was the dangerous moment. Caspian gave them no time to think.
"Where is the captain?" he asked.
"I am, more or less, if you know what I mean," said a languid and rather dandified young person without any armour at all.

Governor Gumpas defaults to bureaucracy as a means of inertia and to generally slothful activity:

He glanced up as the strangers entered and then looked down at his papers saying automatically, "No interviews without appointments except between nine and ten p.m. on second Saturdays."
…
"My Lord," said he, fixing his eyes on Gumpas, "you have not given us quite the welcome we expected. We are the King of Narnia."
"Nothing about it in the correspondence," said the governor. "Nothing in the minutes. We have not been notified of any such thing. All irregular. Happy to consider any applications — "

Governor Gumpas is derelict of his duty to Narnian and cares little about it:

Firstly I find no record that the tribute due from these Islands to the crown of Narnia has been received for about a hundred and fifty years."
"That would be a question to raise at the Council next month," said Gumpas. "If anyone moves that a commission of inquiry be set up to report on the financial history of the islands at the first meeting next year, why then..."
"I also find it very clearly written in our laws," Caspian went on, "that if the tribute is not delivered the whole debt has to be paid by the governor of the Lone Islands out of his private purse."
At this Gumpas began to pay real attention. "Oh, that's quite out of the question," he said. "It is an economic impossibility — er — your Majesty must be joking."

More importantly, Gumpas has neglected the morality and laws of Narnia; showing a complete apathy to the regulations he is employed to uphold:

"Secondly," said Caspian, "I want to know why you have permitted this abominable and unnatural traffic in slaves to grow up here, contrary to the ancient custom and usage of our dominions."
"Necessary, unavoidable," said his Sufficiency. "An essential part of the economic development of the islands, I assure you. Our present burst of prosperity depends on it."
"What need have you of slaves?"
"For export, your Majesty. Sell 'em to Calormen mostly; and we have other markets. We are a great centre of the trade."
… "I believe I understand the slave trade from within quite as well as your Sufficiency. And I do not see that it brings into the islands meat or bread or beer or wine or timber or cabbages or books or instruments of music or horses or armour or anything else worth having. But whether it does or not, it must be stopped."
"But that would be putting the clock back," gasped the governor. "Have you no idea of progress, of development?"
"I have seen them both in an egg," said Caspian. "We call it Going Bad in Narnia. This trade must stop."
"I can take no responsibility for any such measure," said Gumpas.
"Very well, then," answered Caspian, "we relieve you of your office. …”

Thus, @waggawerewolf27, your assignment of ‘sloth’ to Lord Bern (though I think I would assign this sin to ‘The Lone Islands’ rather than merely to the Lord, himself) is very much reflected in the text.
In Into the Wardrobe, the presenter suggests that the Lord Octesian and Eustace both succumb to the sin of pride.  I think we do not know enough about the Lord Octesian to validate what sins he may have been guilty of before he was turned into a dragon.  We do not even know if he was cursed by the dragon’s hoard in a similar manner to what Eustace was.

In your scheme, you assign Octesian to greed.  Certainly, Eustace is behaving greedily when he was turned into a dragon.

He had turned into a dragon while he was asleep. Sleeping on a dragon's hoard with greedy, dragonish thoughts in his heart, he had become a dragon himself.

Assuming Octesian was also attracted to the treasure and enchanted like Eustace, then this assignment makes complete sense.

Again, I think I would assign ‘greed’ and ‘liberality’ to this section of the Voyage of the Dawn Treader as a whole, rather than to this specific Lord as I do not know enough about him and his circumstances.  Eustace succumbs to greed and has to learn to be more generous in his dealings.

If I can cheat.  I would see the encounter with the Sea Serpent to be partly a tag on Eustace’s story.  But it is also a story where both Eustace and Reepicheep show great courage in how they encounter the sea serpent.  Reepicheep does not only show courage, but he encourages the whole crew to team together to defeat the sea serpent.  The difficulty with this approach is that there is no obvious sin in this section of the book.

Both @waggawerewolf27 and Into the Wardrobe suggested that Deathwater relates to greed. (If I understand the presenter of the youtube video rightly, he implies that Lord Restimar might have been dragged into the water by his sword. The book explicitly says that the Lord’s sword was left on the bank by the water’s edge, so this assertion cannot be true.) Nonetheless, Caspian is clearly tempted by greed when he wants the pool to be kept secret for his own benefit. I have already assigned the sin of greed to the events on Dragon Island. If it’s allowed, I would also assign them to this Lord / event here. If it is not allowed, then pride and wrath are both possibilities. Caspian and Edmund begin to behave proudly with regard to each other; arguing over who should be considered as the authority in this situation. In their argument, the two of them also engage in wrath, becoming angry.
Again, if I am allowed to cheat, I would agree with the second Into the Wardrobe video that although there is no Lord on the Island of the Duffers, Lucy is tempted on this Island.  Lucy’s temptations both relate to vanity; she wants to be beautiful beyond the lot of mortals.  The degree to which she becomes beautiful in this spell is downright dangerous, with men of all nations fighting uncontrollably to make her their queen.  Likewise, when she wants to know what her friends really think about her, the reason seems to be for her own self-gratulation / vanity.  In Into the Wardrobe

they assign the sin of ‘vain-glory’ to this incident.  This feels spot on to me given the events on this island.

For the Dark Island / the Lord Rhoop, you suggested ‘lust’ as the sin – which makes sense as the crewmen talk about how their dreams will be fulfilled on this island and Lord Rhoop comments that it was talk like that which first brought him to this island.  These people would be coveting what did not belong to them.  In Into the Wardrobe, he comments that Dr Martin ascribed “Envy” as the sin of Lord Rhoop because the crew are coveting something that really belongs to someone else.  The host, in the 2nd Into the Wardrobe video, suggests that the actual sin is sorrow / despair.  I thought ‘despair’ is the main temptation in this section and hope would be the opposing virtue, so this assignment fits well.

I struggle to assign any sin on Ramandu’s Island.  The few words each lord speaks suggests a sin for each (Mavramorn: ‘mustard please’ – gluttony, Argoz: ‘out oars for Narnia’ – sloth and Revilian: “Weren't born to live like animals.” – wrath).  Their backstory seems to indicate that these assignments work as well; “Here is the good place. Let us set sail and reef sail and row no longer but sit down and end our days in peace!”  Given that this man stated this at a banquet table and his first words from his sleep was ‘mustard please’, it seems like he is perhaps overly fond of food.  But he could also be guilty of sloth; he is supposed to be on a quest to find the lands on the Eastern seas and he wants to stop with his quest unfulfilled.  The second man seems to have a slothful attitude: “No, let us re-embark and sail for Narnia and the west; it may be that Miraz is dead.”  He too wants to abandon their quest.  The third man’s words sound almost lustful in his desire for adventures: “No, by heaven. We are men and Telmarines, not brutes. What should we do but seek, adventure after adventure? We have not long to live in any event. Let us spend what is left in seeking the unpeopled world behind the sunrise.”  The three quarrel and Lord Revillion grabs the stone knife to fight his friends.  This is wrathful behaviour on this Lord’s behalf – and arguably on behalf of all three.

When we look at the heroes of our story, they do not seem to be tempted in any great way on this island.  Arguably the crew of the Dawn Treader were tempted by sloth to sail no further and enjoy the food at Aslan’s table; and the sailor who stayed behind at Ramandu’s island was guilty of sloth.  As such, I would assign ‘sloth’ to these events, if I am allowed to double dip again (as sloth was already assigned to The Lone Islands).  Otherwise, I would assign ‘wrath’ as the sin present in this sequence.

During the journey to the end of the world, Caspian is tempted to neglect his duty as king and to travel with Reepicheep to the world’s end.  Again there are a number of sins that could fit here: Caspian could be lusting after adventure, he could be ‘greedy’ for adventure, he could be showing pride “who says ‘can’t’ to a king”, he could be manifesting sloth in neglecting his duty, he manifests wrath when he loses his temper and walks out on the crew.  After encountering Aslan, Caspian calms down, he diligently accepts his duties as king, Aslan has humbled the king, he allows the Pevensies and Eustace to leave, thus he repents of whichever sin is assigned and manifest the opposite virtue.

Anyway, that was my attempt at The Voyage of the Dawn Treader.

Grasshopper pie?

https://www.flickr.com/photos/94921703@N00/3266470174

I watched several old films here including Star Trek: The Motion Picture and Gandhi. A local college has renovated the building and it serves other purposes now, e.g., as an auditorium, although it still can show films. It has a different name now.  It is no longer named after the town in which it is located, Holland, Michigan.