H20: Just Add Water
Available free on Hulu.
Three teenage girls find an ancient cave and while exploring, undergo some serious changes. Try making it through your teenage years, with all the trials and tribulations, as a mermaid. Can't be easy.
Available, for the moment, on YouTube... catch them while you can.
A boy manages to touch the sacred waters of Moon Pool at a full moon and ends up a merman, with a new tail and new life. Three mermaids, guardians, are sent from their home to steal back his accidental powers. If they don't, they are out on their butts. A adaptation of H20, but not as good (H20 addresses more than just mer-teen life).