Yes, you could blame the script for veering too far from the book. And I would add that the quality of the dialogue was just sub-par, even poor in several places. That's unfortunately the case with all of the Walden films, but at least Wardrobe and Caspian kept enough of the original themes intact and so it was easier to ignore the jokes that missed their mark (Ramandu's daughter "changing form" line comes to mind) or the bland modernizing of Lewis' original language. Why did the Markus/McFeely/Petroni team feel the need to rework so much of the book dialogue?

Also, it was clear they didn't understand or didn't trust that Dawn Treader had a compelling enough plot for a big budget fantasy film, otherwise they would have embraced the episodic nature of the story and found ways to pull together the plot by uniting the character struggles and life lessons during the adventure. For instance, when Edmund and Caspian confront each other about the treasure found at Goldwater Island, the writers could have come up with a way to develop Caspian's longing for notoriety and prestige long before they reached that location. Perhaps Caspian feels overshadowed not only by Miraz in his past, but now also Edmund and Lucy, his royal successors trying to hold him back from taking credit for the riches they have just discovered.

It's true that the book has a lot more emphasis on the emotional and mental challenges the main characters face, not an easy thing to show on film, but the way they tried to solve those issues was lazy and uninspired, and as you already pointed out, very unfaithful. While I don't think it would have necessarily improved the box office earnings, much better writing and a more confident, book-inspired plot would have made a big difference.

For me, there were two major and yet minor things they could have done to make the film a whole lot better - one get rid of the whole green mist and seven swords for the seven lords/saving the world of Narnia thing, and two, just sticking to the heart of the books - making it a fun, nautical adventure.  This could have been fairly easily done and possibly may have made the film cheaper to make, or at the very least simpler.  It's a bit disappointing, because I love the start of VDT - it feels like the start of a fun nautical adventure, but by the time they reach the Lone Islands (in the film) it quickly descends into a whole saving the whole world thing and of course at that point massively diverts from the storyline of the book.  I also like many of the aspects of the last few scenes of the book (although the reference to Eustace's "friend" Jill was a bit unnecessary- as of course she wasn't a friend at that stage according to the books).  And I LOVED the credits tribute to Pauline Baynes.  All in all, I reckon if they had made those few changes, kept closer to the book and made the film feel more like a fun nautical adventure story rather than the need to save the whole world of Narnia - I reckon they could well have made at least one more film, possibly all the rest.

The biggest problem with the VDT movie was not that they added the Green Mist... It was that they added a whole bunch of other stuff to the original script, and then we're forced to replace it all with the Green Mist as a much cheaper alternative when they changed studios and were forced to slash the budget 

So for example, in the original VDT script, the slaves of the Lone Islands are all lured into a giant cave in the shape of a snake which makes them disappear.... In the final script they are simply put on a row boat and some green mist comes along ... Not saying the former is necessarily better from an adaptation perspective, but clearly the final movie is actually drastically different to the one the writers set out to make.

As noted by others, the screenwriters clearly felt the need to add a stronger through-line to the movie in order to connect all the dots, and to insert a villain to provide a more compelling motivation for the quest. You can actually see this sort of writing philosophy fully articulated by the screenwriters themselves in this podcast interview - one they themselves define as an extreme "structuralist" approach to storytelling:

https://youtu.be/XNXc34NKsT0?si=VHcyjzHRjC14p8As

As to the overall question of how you could make the movie better - the example I always look to in this instance is David Lowery's "The Green Knight'" (2021)

It's a highly episodic movie (each episode even has it's own title card) and on first viewing it may not even appear to have any connective tissue at all... But once you come to understand the core theme, it becomes clear how and why all the elements link together. It's a film which therefore hugely rewards repeat viewings, and it's clear to see why critics adored it.

Therefore, following the Green Knight's lead the answer to the question is that rather than making the connective through-line of the movie be an external threat which drives the narrative along, you make it an internal character motivation which drives the characters along.

I think for VDT you would probably therefore need to focus a bit more on Caspian's inner emotional journey - the singular drive and purpose behind his quest.

It may have been a great or at least a very good movie if they had been more faithful to Lewis’ own story. I don’t know how much control Michael Apted had over the story adaptation. But what he did visually to the film probably would have worked much better with a story that was closer to the book. Visually the film was quite good because of his talent as director.  It wasn’t essential to add unnecessary things to the story.  There wasn’t a need for the Green Mist or the little girl stowaway on board. The voyage was interesting enough just as it was in the book. And other adaptations such as Focus on the Family radio drama and the BBC series did not have to change the story very much to make it interesting for listening or watching.

@narnian78 The Mist was what the production conceived of, after Mr Gresham suggested something to signal each time someone was tempted. He thought of a sound, maybe a bell. They made it a big thing that was the colour of the witch in the next book, and swallowed up boatloads of people. (Of course, Lewis didn't emphasise temptation but bad dreams).

I think Walden's version of VDT overdid the Jadis factor. It seems as if they wanted to retain Tilda Swinton as long as possible & were struggling to give her something to do. The question was when VDT was released, would they do SC first or MN? BBC had already made the mistake of following VDT with SC, when Barbara Kellerman became the Lady of the Green Kirtle. After that, neither Barbara Kellerman who had played LOTGK, nor Tilda Swinton, could play Jadis in Magician's Nephew.  

@waggawerewolf27 

I always wondered what the White Witch was doing in the Dawn Treader  movie.  Although it is true that the Dark Island had nightmares there is no mention of Jadis on the voyage there according to the book. They should have left her out and provided another nightmare which already in the book or something similar to it. There was enough of a dark experience in the original story so that it would not have been necessary to add anything else to it.  And the White Witch was supposed to be dead when Aslan defeated her in The Lion the Witch, and the Wardrobe. There was no reason for her to appear again even if it was only in Edmund’s imagination. 

The one BBC Miniseries adaptation of the books that impressed me at the time was Voyage of the Dawntreader.  Having a TV episode for each portion of the book worked (in my mind) for reflecting the episodic nature of the original material while preserving the overall narrative.

I don't think this is a book where you can create a simple 1st-act, middle-act, third-act structure and adapt it faithfully.  @icarus suggestion of something akin to "The Green Night" is a good suggestion ("Inglorious Bastards" is another example of a movie with a more episodic plot line). I don't think embracing a less traditional narrative structure is something often attempted with children's books though - so I doubt you'd ever get a studio to sign off on it.

(And a tangent - to be controversial, I think part of the problem with adapting "Prince Caspian" is also that C.S. Lewis did something that is uncommon to movies in that he begins with a mystery and then uses an extended flash-back to explain the mystery. Adaptions always switch the order so that we begin with Caspian's backstory and then show the mysterious "where are we and what's going on" chapters when the audience already knows what the Pevensies don't and for us there is no mystery. I would love to see an adaption where we follow the initial story from the Pevensies perspective and as an audience we don't know what's going on either.  They'd probably have to adjust Trumpkin's introduction so that there are stakes during the flashback sequences if someone attempted this - I.e. we shouldn't be told that characters are currently alive before we get a flashback in which much of the drama is about whether or not they will survive.)

So I wasn't going to really join in on this discussion or the one about the other Walden Media Narnia movies, not because I don't think they're interesting to discuss. I do and I have strong feelings about them. But...I've been reading this forum a long time and I feel like I pretty much "get" every member's takes on them (except for our newest members, of course.) Also, last year, I did a really lengthy series on my blog analyzing those adaptations scene by scene, so I feel like I've had my fill of the subject.  

But then I remembered this particular criticism I made of the Deathwater scene from The Voyage of the Dawn Treader (2010.) It has to do with the acting, but I don't blame the actors. I blame the director. This isn't the worst scene, but I think it demonstrates my overall problems with the film's direction. 

Now it tends to bug me when critics disparage works of art by saying the artists just made it for a paycheck and didn’t really care about the quality. How do they know that? It seems to discount the possibility of artists trying to make great art and failing through a lack of taste. For that matter, does it ever occur to the critics that the works of art they love and into which they believe the artists put their hearts and souls might have actually been made by artists who were just going through the motions but had enough talent to make something great anyway? How do we know the critics aren’t just projecting their own feelings about the art onto the artists? So I try to avoid making those kinds of statements on this blog but it’s hard to resist with these Narnia movies because…I really do think Andrew Adamson, the director of the first two, put a lot of passion into them whereas Michael Apted, the director of this third one, didn’t. Mind you, I don’t consider everything aboutThe Lion, the Witch and the Wardrobe(2005) orPrince Caspian(2008) great, but I believe that Adamson thought they were great and was trying to make the greatest movies he could even when I disagree with his taste. With Michael Apted, well, I’m sure he wished to avoid making an outright bad movie. Sheer professional pride forbad him that. But I don’t get the impression he was trying to make the greatest movie possible. It feels like he just wanted to wrap up filming as quickly as possible every day so he could go home and do his laundry, and that attitude trickled down to the whole crew. Well, not the whole crew. Some people did great work on the movie, but I imagine the credit goes to those individuals, not to Michael Apted’s leadership.

And the nice thing about this scene is that it allows me to give specific reasons for this impression beyond just “I liked the other movies better.” Caspian and Edmund are supposedly going through the same experience here, but their actors portray it entirely differently. Skandar Keynes snarls and hams it up like he’s transforming into Mr. Hyde. Ben Barnes, on the other hand, gives a subtly creepy performance, making it hard to say when exactly Caspian starts going crazy. (To be fair, that may just be because his dialogue is relatively less heavy handed.) It feels like Apted had no particular vision for the scene and just let the actors do whatever they wanted. If you ask me, he should have been telling Keynes to follow Barnes’s example.

Here's a link to that particular post but you've just read what is probably the most interesting part of it. The Voyage of the Dawn Treader (2010) Part 7: This Place Has Tempted You! | The Adaptation Station.com

I think they were probably in too much of a hurry when they made the Dawn Treader movie. They probably should have taken more time with the script to ensure better accuracy to the book. The visual effects were quite good and the movie is quite watchable because of them, but a better script that was not rushed would have made them much better.  I have never disliked the film, but I think it had the potential to be something so much better. They had a much higher budget than the BBC production so they were not as limited by money. And there also were two other productions with good faithful stories, e.g. the BBC series and Focus on the Family.   So there was no reason why a faithful production couldn’t be done.  I don’t think it would have cost much more to have a better script with ideas already provided for in the original book.  I guess the old saying applies that if isn’t broken don’t try to fix it.  C. S. Lewis already got the story right, and there wasn’t that much that should have been changed for adaptation.