Los Angeles County coroner's Lt Larry Dietz said yesterday the man was taken from Venice Beach and pronounced dead at a hospital.
Nine people went to hospitals, including one who remained in critical condition.
The lightning struck as rare summer thunderstorms swept through Southern California.
Steve Christensen said his friend tried to help when lifeguards began searching for a missing swimmer.
"He went out to the water to find him and walked right into him," Christensen said. "He was face down on the bottom."
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Stuart Acher said he was struck while playing volleyball on the beach.
"All of a sudden there was a big flash of light and a boom, and it felt like someone punched me in the back of my head," he told KABC-TV. "It went down my whole side of my right body, and my calves sort of locked up, and I fell over.
And I looked up and everybody else was, you know, falling over."
Other fire officials said most of those taken to hospitals were mainly shaken up and were expected to recover.