Thirty four people died after a boat packed with scuba divers caught fire near an island off the Southern California coast and authorities have called off search efforts for survivors.

The Coast Guard and police said no one has been found alive after flames tore through the dive boat early on Monday as passengers on a recreational scuba diving trip slept below deck.

Investigators have not yet determined how the fire broke out.

Santa Barbara county sheriff Bill Brown said the bodies of 20 victims have been recovered and divers have seen between four and six others in the sunken wreckage.

He said authorities are trying to stabilise the boat that sank in about 60 feet of water so divers can recover those remains.

Boat Fire California
Divers with the San Luis Obispo County Sheriff’s Dive Team prepare to search for missing people (Christian Monterrosa/AP)

One passenger, marine biologist and veteran diver Kristy Finstad, 41, was identified in a Facebook post by her brother, Brett Harmeling of Houston.

“Please pray for my sister Kristy!! She was leading a dive trip on this boat,” Mr Harmeling wrote.

The missing and dead were among 39 passengers and crew who had departed Santa Barbara Harbour on Saturday aboard the boat for a Labour Day weekend trip.

The fire broke out about 3am Monday as the Conception was anchored off Santa Cruz Island, about 90 miles west of Los Angeles.

The crew appeared to quickly call for help.

“The call was garbled, it was not that clear, but we were able to get some information out of it to send vessels,” Coast Guard Petty Officer Mark Barney said.

Boat Fire California
Orlando Aldana, 42, of Santa Barbara, lights candles in honour of the victims (Stefanie Dazio/AP)

By the time it got there around 5am, a Coast Guard helicopter and a fireboat were on the scene.

Capt Amaral said he first searched the water and shoreline, then turned back to the Conception, which was adrift.

He attached a line and pulled it into deeper water so the fireboats could reach it.

“We launched that boat knowing that the vessel was on fire, lots of people aboard,” he said.

The five crew members, meanwhile, went on a dinghy to a private fishing boat, The Grape Escape, that was anchored near the north shore of Santa Cruz Island.

Two had minor injuries.

Boat Fire California
A dive boat is engulfed in flames (Santa Barbara Fire Department/AP)

“When we looked out, the other boat was totally engulfed in flames, from stem to stern,” Mr Hansen said.

“I could see the fire coming through holes on the side of the boat. There were these explosions every few beats. You can’t prepare yourself for that. It was horrendous.”

Mr Hansen said two of the crew members went back toward the Conception looking for survivors but found no one.

The 75-foot Conception was on a three-day excursion to the chain of rugged, wind-swept isles that form Channel Islands National Park in the Pacific Ocean west of Los Angeles.

The fire broke out as the boat sat anchored in Platt’s Harbour off Santa Cruz Island.

The Conception, based in Santa Barbara Harbour on the mainland, was owned by Santa Barbara-based Truth Aquatics, founded in 1974.

A shrine outside Truth Aquatics in the Santa Barbara Harbour grew Monday night as mourners came to pay their respects.

Dave Reid, who runs an underwater camera manufacturing business with his wife, Terry Schuller, and has travelled on the Conception and two other boats in Truth Aquatics’ fleet, said he considered all three among the best and safest.

“When you see the boats they are always immaculate,” he said.

“I wouldn’t hesitate at all to go on one again.

Boat Fire California
A shrine outside Truth Aquatics for the victims of the Conception boat fire (Stefanie Dazio/AP)

His wife said Truth Aquatics crews have always been meticulous in going over safety instructions at the beginning of every trip she has been on.

“They tell you where the life jackets are, how to put them on … the exits, where the fire extinguishers are, on every single trip,” said Ms Schuller.

“They are the best, the absolute best.”

Both said the sleeping area is comfortable but tight, however, with bunk beds stacked next to one another on the lowest deck.

Coming up to the top deck to get off requires navigating a narrow stairway with only one exit.

Boat Fire California
This January 2019 photo tweeted by the Santa Barbara County Fire Department shows the dive-boat Conception returning to Santa Barbara Harbour (Mike Eliason/AP)

Coast Guard records show all safety violations from the last five years were quickly addressed by the boat’s owners.

Some violations were related to fire safety. A 2016 inspection resulted in owners replacing the heat detector in the galley and one in 2014 cited a leaky fire hose.

The Conception was chartered by Worldwide Diving Adventures, which says on its website that it has been taking divers on such expeditions since the 1970s.

Andy Taylor, owner of Blue Water Hunter Dive Shop in Santa Barbara, said he discussed dive conditions with several people on Friday as they were buying some last-minute things before boarding the Conception.

Boat Fire California
Flowers are placed at a dolphin statue (Ringo H.W. Chiu/AP)

He said he was on the phone all day Monday as friends checked to make sure he had not been on the boat.

“Right now it’s a big question of who was on there and who wasn’t,” he said.

“I’m scared to see the list of names, honestly.”