What's the name of the film in which a boy turns into a mermaid?

Question

I can't remember much at all about this film, I saw it when I was very little. What I do remember though, is this guy being really concerned, because he thinks that nails are growing all over his body- that the ones you get on your fingers, and I remember shouting out (to my siblings) "Those aren't nails, they're scales!" Umm, other details, there was a scene with the guy and someone in a wheelchair (his dad??) on a wooden pier thing, and then the guy jumps into the water. Also, I think his muother was a mermaid.

Could anyone help? It's been really bugging me, I want to see it again. And no, it's not Splash.

Answer 1

The 13th Year

is a Disney Channel movie

Answer 2

The Thirteenth year.  

Answer 3

I'm not sure but I think you're talking about The 13th Year.

Answer 4

The Thirteenth Year.

It really scared me as a child.