I immediately could think when I saw Necille with the Gillyweed that he will give it to Harry instead of Dobby as I have heard that they cut Dobby from the film, and Harry must have his gillyweed from someone. I think that they make up the scene in the chamber of secrets film where Neville fainted because of the mandrakes. In the book that did not happen!
The portrait of the mermaid I had imagined different, but it looks quite well.