My sister remembers seeing a movie at the theater when she was around 8 or 9 but has never been able to remember what movie it is and would like to know show she can see it again.

It's about a boy who lives on some islands with his father and he loved to swim. There's some sort of conflict and his father ends up dying. The boy ends up being able to stay under water for longer and longer periods and his skins starts drying out and bleeding whenever he's away from the water for long periods. She says she remembers a scene where he has a nightmare and wakes up with his ears and nose bleeding because his skin is dry and cracking. He somehow develops some sort of gills because he ends up being able to breathe under water and he ultimately ends up living under water.
Also, there was a girl that he was friends with and my sister thinks there was some sort of love interest between them but there was some sort of conflict and they couldn't be together, partly because she lived on land and he ended up being mostly in water.
The movie also focuses on greed and men who work in some sort of business dealing with ships and she remembers there being some sort of explosion and the main character goes into the water to escape from it and he uses the ocean as basically an escape from the world.
She doesn't remember any of the actors' names. She said the main character had black hair and he was cute. She's tried asking our parents about the movie but they only vaguely remember it and she's tried asking other people and looking for it online but can't find it anywhere. Does anyone know anything about this movie? If so, please let me know. Thank you!

The Incredible Mr. Limpet. With Don Knotts