Fresno County Sheriff's Office investigators say the man was part of a group of four adults who were out on a motorized boat near the area of the Deer Creek Marina.
According to witnesses, the victim jumped and out of the water several times to swim recreationally. The last time, just before 1 am on Saturday, he did not resurface.
He was not wearing a life jacket, and he was not seen again.
Patrol teams have been searching the lake all morning long, deploying boating units, air resources, and drones.
However, they haven't found the victim.
The area where he went under is about 200 ft deep, authorities say - too deep for dive teams to investigate.
At this time, no foul play is suspected, but the accident is under investigation.
The lake is open 24/7, but some people swimming in the water on Saturday told Action News it's not common others are on the water that late in the night.
Officials say if you're in or near the water, wear a life jacket because it can be the difference between life or death.