The Deep End(2006)

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.