The Thirteenth Year (1999)

Cody Griffin is an adopted teenager; his birth mother is a mermaid who left him on the Griffins' boat when he was a baby to avoid being captured. As she left, she was spotted by Mr. Wheatly, a fisherman who became obsessed with finding mermaids. Years later, Cody establishes himself as a quick swimmer on his school's swim team, and he has a girlfriend named Sam. He is under a lot of pressure from a big swim meet coming up and because he is failing biology, he is partnered with Jess Wheatly, the class geek who is a marine biology expert.