The Los Angeles County Medical Examiner's Office identified the drowning victim as 56-year-old Fredrico Salguero.
The incident happened about two miles offshore from the Avenue C lifeguard tower at about 1 p.m. Sunday.
Authorities responded to an overturned 26-foot MacGregor boat - a dual-purpose boat that can operate as a sailboat and a motorboat - where five people were clinging to the side, including a child and the boat's captain.
But a sixth person who had been on the boat had not surfaced and was believed to be trapped inside. Rescue divers found the victim after searching underwater for about 45 minutes. He died at the scene.
Lifeguards and other people familiar with the area say the wind conditions on that day were not ideal for small boats.
"We've had pretty windy conditions since the jump this morning," said Capt. Pono Barnes with Los Angeles County fire lifeguards. "It was pretty choppy on the water. We have a single flag today. It was a gale warning. If you are planning on sailing or going out on the water, I'd maybe re-evaluate and wait for calmer conditions."
"They go out in their boats. They don't really know nothing about the navigational aspect of the ocean's conditions -- how nasty they can get out there. And it comes up fast out there when it comes out," said Bill Schwaneberg of Marine Tech Engineering.
The investigation continues but so far it's unknown how the boat overturned in the first place.