Ocean Butterfly (2006)

Original title: ผีเสื้อสมุทร

An unusual fantasy love story of Wan, a girl who's born with the supernatural ability to stay underwater. While visiting Butterfly Island, Wan discovers her special ability to dive and stay under deep sea. She also meets Tan, a photographer who always helps her when she has problems, and with whom she eventually falls in love. Wan soon realizes that her new-found powers come with a price; she's not a normal girl. Her body will be transferred into a kind of creature when she gets into the water. Once again, it's Tan who must help Wan figure out what's going on with her body without knowing that a mysterious secret awaits him under the sea.