According to the Riverside County Sheriff's Department, a family of four was on a privately owned boat on the north end of the lake when it overturned about 1:30 p.m.
A sheriff's boat was on scene within minutes. A mother and her two adult children were rescued and were unharmed, but a 50-year-old man could not be found.
Two dive teams searched for the missing person but were unsuccessful. They are now searching for his body.
It was not clear what caused the boat to overturn.