The Thirteenth Year, on Disney Channel
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It starts with this fisherman named John Wheatley spotting a mermaid, and trying to catch her. She's carrying a baby, whom she puts in some other boat in the area, and when she goes back, the baby has already been found and taken in by a couple, Whit Griffin (who's just starting a tour boat business) and his wife Sharon. Why the mermaid left the baby there is unclear to me... at first it seemed like she set him down so she could move faster while evading Wheatley, but I dunno. Seemed like she came back to look for the baby before really completing said evasion. Maybe she wanted the baby to be taken in, I'm not sure... but either way, she seemed sad to see him go.
The movie then jumps forward 13 years, right around the birthday of Whit and Sharon's son, Cody. He's on his school's swim team, though he always seems to come in second to this other guy named Sean, who's kind of a jerk. You know, one of those popular jocks who everyone likes. Cody seems pretty much the same way, actually. But there's this sort of nerdy kid named Jess Wheatley (yes, the son of the fisherman), and he and Cody are assigned as partners in a marine biology project. At first Cody isn't really happy to be working with him, but they soon become more friendly (which really annoys Sean, because Jess isn't "one of them"). Jess has never learned to swim, though he loves marine biology (which doesn't interest Cody). So, Cody will teach him to swim. Oh yes, and Cody also has a friend, or sort of girlfriend I guess, named Sam.
Anyway, weird stuff starts happening to Cody. He's having dreams about swimming underwater, and he starts drinking a lot, and he sometimes gets scales on his hands when they get wet, and sometimes produces a great deal of static electricity (you know, like eels?). And so he's freaking out, and trying to keep this secret from everyone. But Jess finds out and starts doing research and comes to the conclusion that Cody is a merman. And Cody's parents freak out, and want him to stay away from the water (which sucks for someone on the swim team, eh?) And when Sam finds out she freaks a little, but is more upset that Cody didn't tell her about it. She gets over it, though.
Welp... Jess's dad has been mocked for the past 13 years because he told everyone he had seen a mermaid. And now that Cody's merman nature is starting to assert itself (as apparently happens with merfolk who've lived on land, when they turn 13), his mom comes back, and so Wheatley tries to catch her again, to prove to everyone that he's not crazy. But... well, I don't know that I should say any more about what happens in the movie. Anyway, it's all sort of simple and predictable. I didn't really think the writing or acting were that great. But it wasn't really a bad movie. Worth having seen once, anyway.