I say she is a water nymph. A sea girl is something else.

In the movie the Sea People were just like people, but in the movies they are shown as mermaids. But I am pretty sure she is a Sea Girl, because the ones they see in the book were Sea People, and the Water Nymphs lived in the rivers not the sea. And I also think that the Nymphs were the spirits of of the rivers just as the Dryads were the spirits of the trees.
Also in the book when they see the Sea People in VDT they reference the Sea People that sang at Cair Paravel, and in the LWW movie they are portrayed as mermaids as is the girl in the trailer

Actually there is no way she could be a nymph because in MN the sons and daughters of Frank and Helen married the River nymphs which would be very hard if they could not leave the water

Actually, according to Narnia.com, the person that Lucy waves to in the trailer is a water nymph.

Then they have their terms mixed up. because it is very clear in the book that it is a Sea Person

These are the people that made dryads into floating clouds of flower petals, why would anyone be surprised they got mermaids, sea people, and naiads mixed up?

^I'm not surprised . Dissapointed? Yes.

Yes, I believe it's a naiad...I think sea people would have been better, but it's not a huge disappointment to me...Who knows, maybe the sea people will show up later?

These are the people that made dryads into floating clouds of flower petals, why would anyone be surprised they got mermaids, sea people, and naiads mixed up?

I can understand people's objections to these interpretations, but at the same time they are very visually appealing and a creative way to interpret an old idea. I think it is important to continually take new ideas on older concepts. I think that this a VERY small detail that does not take away from the story. It also does not take away from the written book.

It might not take away from the book, but have you forgotten the children of Frank and Helen marry tree and water spirits? So now they'll either have to eliminate that or come up with a forced explanation for how flower petals can have babies with humans. This is an example of a change made "BECAUSE IT LOKKS PWEETTYY!!11" that is made with no thought to how it will affect future movies.

Then they have their terms mixed up. because it is very clear in the book that it is a Sea Person

Well the immediate reason for making the dryads into wispy wind flower people, and the sea people into sea nymphs that are watery is to do away with the implied (and in one Paulin Baynes illustration, explicit) nudity of those mythical peoples.

What I was saying was that I think the Nymphs were the spirits of the rivers, while the Sea People were just people that lived in the water.

Actually, nymph just means a minor nature goddess. A naiad is a type of nymph that lives in rivers.

They could just put clothes on the spirits and sea people. That's what Disney did to the Little Mermaid and I doubt anyone thought it ruined the story to have her wearing a top.

Were the Sea People supposed to be Nymphs? or just people.

I always assumed they were just underwater-living people. Not humans, obviously, but something similar.