It happened at Cricket Hollow Park in Reedley.
Police say the 24-year-old victim tried to swim across the river from Reedley Beach, but went under near Olsen Bridge.
After losing sight of him, family members flagged down a passing boater who spotted the man face down in the water.
The man was pulled to shore and emergency crews performed CPR.
He was then taken to a nearby hospital, where he was pronounced dead.