Authorities have recovered 33 bodies and say one is missing from the sunken wreck of a US dive ship after a fire, with DNA to be used to identify the dead.

The new count of confirmed deaths came after officials recovered 13 bodies on Tuesday, Coast Guard Lt Zach Farrell said.

The search for survivors was called off on Wednesday.

Previous estimates put the death toll at 34 after a pre-dawn fire ripped through commercial diving boat Conception while victims slept below decks.

Five crew members, including the captain, managed to escape. The vessel eventually sank and overturned, making the recovery of bodies challenging.

The victims identified so far include high school students, a science teacher and his daughter, and a family of five celebrating a birthday.

Diving instructor Kristy Finstad, whose company Worldwide Diving Adventures chartered the boat for the three-day tour, was also among those recovered from the water.

Officials said the captain and four other crew members who survived jumped off the front of the vessel, swam to an inflatable boat at its stern and steered it to a ship anchored nearby.

But flames moved so quickly through the 23-metre Conception that it blocked a narrow stairway and an escape hatch leading to the upper decks, giving those below virtually no chance of escaping, authorities said.

Authorities ended the search for any other survivors on Tuesday. DNA will be needed to identify all the victims.

Authorities will use the same rapid analysis tool that identified victims of the deadly wildfire that devastated the Northern California town of Paradise last year, Santa Barbara County Sheriff Bill Brown said.

Mr Brown said he had heard anecdotally that those who died ranged from teenagers to people in their 60s.

The fire broke out shortly after 3am on Monday as the boat sat anchored in Platt’s Harbor off Santa Cruz Island, among the rugged, wind-swept isles that form Channel Islands National Park in the Pacific Ocean west of Los Angeles.

– with AAP