The Orange County Sheriff's Harbor Patrol said the man went for a swim with a group about a half-mile off Newport Beach on Sunday.
At one point, someone backed the double-decker boat toward the man when he was ready to climb aboard.
That's when he got caught in the propeller.
People on the boat tried to give him CPR, but he was pronounced dead at the dock.
Deputies said alcohol did not appear to be a factor in the incident. The investigation was ongoing.
The Associated Press contributed to this report.