Tweets from the Coast Guard in Los Angeles said there were reports the people were on a boat that was on fire near Santa Cruz Island.
It said multiple rescue assets have been launched from local agencies to assist the 30 people on board on the 75ft yacht.
A group of crew members have so far been rescued, one with minor injuries, and efforts continue to evacuate remaining passengers.
The Ventura County Fire Department confirmed 34 people have died, KTLA in Los Angeles reported.
However the Coast Guard has not confirmed any fatalities.
The Daily Beast also quoted Captain Brian McGrath of the fire department as saying there were deaths but he could not give a number because the Coast Guard were still searching the water.
He reportedly said that five people had been rescued and that 34 people were still unaccounted for.
Santa Cruz Island is off the coast of Santa Barbara.
https://news.sky.com/story/rescue-under-way-as-boat-on-fire-with-30-on-board-in-southern-california-11800691