The Redondo Beach Harbor Patrol received a distress call at about 1 p.m. Sunday.
Authorities were called out for an overturned 26-foot MacGregor boat - a dual-purpose boat that can operate as a sailboat and a motorboat - about two miles off the coast where five people were clinging to the side, including a child and the boat's captain.
They were able to rescue the five people and bring them back to shore for evaluation. They were described as being in stable condition without major injuries.
But a sixth person, a man in his 50s, who had been on the boat had not surfaced and was believed to be trapped inside.
Rescue divers went underwater and after searching for about 45 minutes found the missing person trapped inside the boat. They brought the person to the surface and attempted CPR but it was unsuccessful. The person was declared dead at the scene.
It was not immediately clear how the boat overturned.
Lifeguards said conditions were choppy on the water Sunday and not ideal for boating.
"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."
The Los Angeles County Sheriff's Department is investigating.