The jury in Georgetown, Delaware, deliberated for six hours before returning its verdict against Melvin Morse, 60. Morse was charged with three felonies - two for alleged waterboarding and one for alleged suffocation of the girl, now 12, by hand.
He was convicted of one felony -waterboarding in the bath - and five misdemeanours. Jurors reduced the second waterboarding charge to a misdemeanour and cleared Morse of the suffocation charge.
Morse, who showed no reaction as the verdict was read, was ordered to surrender his passport and will remain on bail until his sentencing on April 11.
He faces a maximum of 10 years in prison but a lesser punishment is likely under state sentencing guidelines.
Each misdemeanour carries a maximum of one year in prison.