Todd Sousa, 36, was arrested on Thursday night as he was teaching a swim class to students at a school in South Orange County.
Authorities said the victim was a student in one of his classes, and the alleged abuse happened for nearly a year.
Police began investigating Sousa after the mother of the victim brought her into a local police station on Wednesday night and stated that her daughter was involved with her swim coach, according to Jim Amormino, spokesman for the Orange County Sheriff's Department.
Sousa and his wife own Swim Venture Inc., a well-known swim club in Orange County. The company's mission is to provide the best "learn-to-swim experience imaginable." He has been teaching and coaching swimming full time since 1994, and the children in the classes are as young as 12 months old.
His club leases pool space at a number of school campuses across South Orange County to teach swimming, but he is not employed by any of the schools.
Officials with his company would not go on camera, but in a statement said, "Our hearts go out to the family involved and [we] wish to convey that the actions of Mr. Sousa were an isolated event and in no way represents Swim Venture as a company, its values and mission statement."
Sousa is also a teacher at AquaZot, a private swim club that teaches all ages and levels of competitive swimmers.
Sousa faces a number of charges including child molestation. He was unavailable for comment.
Authorities are concerned that there may be other victims. Anyone with information was asked to call the Orange County Sheriff's Dept. at (866) TIP-OCSD.