What was the purpose of the White Witch's cameo appearance to Edmund after he, Lucy, and Eustace have come aboard the ship?
They got the picture right, but they are drawn into the sea first when entering Narnia. As I remember from the book they all land on the deck of the Dawn Treader
and don't swim in the water first. I don't remember any direct quotes from the book in the trailer, although it does look like there's some faithfulness to the plot of the story. I think the old BBC specials were better for accuracy (preserving the plot and dialogue) in spite of their low budget.