Aquamarine
After a vicious storm , half the ocean washes into the beach club swimming pool. Two teenagers, Haley and Claire, discover a mermaid in the pool. She tells them that she needs to find love in three days, and they agree to help her because helping a mermaid means you get a wish, and they decide that they can use their wish to try and stop Haley from moving to Australia.
http://www.imdb.com/title/tt0429591/