The Deep End(2006)

Overview:

Lucille, an introverted eleven year old girl, live with her parents, who are caretakers of the swimming pool in which they stay. Because of the weirdness of this place, which looks like a flying saucer, an awful anguish comes out of Lucille's mind. The "unidentified frightening object" of her confusion would happen to be even more obsessing than all fears of childhood.