Waterboarding simulates drowning and has been used in the past by U.S. interrogators on terror suspects. Many critics call it torture.
Sixty-year-old Melvin Morse testified Tuesday in his endangerment and assault trial that the girl sometimes had lice that required hair washing.
He testified Monday that he and the girl's mother became exasperated with washing the girl's hair after she started taking showers instead of baths around 8 years old.
The girl, now 12, and her mother have said Morse used hair washing as a threat or punishment.
He faces more than 20 years in prison if convicted on all charges.
Morse, who has researched near-death experiences involving children, has denied police claims that he might have been experimenting on the girl.