One person has died and two others remain missing after a triple-deck pontoon boat carrying 19 people capsized near Alcatraz Island in San Francisco Bay on Tuesday.

Emergency crews launched a large search and rescue operation after the vessel overturned, with the U.S. Coast Guard joining local agencies in the response. Eleven boats remained on the water for hours as crews searched for the two missing passengers.

San Francisco Fire Chief Dean Crispen said the first police boat to reach the scene found a seriously injured person.

Despite efforts to save them with CPR, the victim died.

Of the 19 people on board, 13 made it safely to shore, three were taken to hospital, one person died and two were still unaccounted for.

"We are still conducting an active search of the area," Crispen said.

"We have 11 vessels on the water conducting that search. We are going to continue for hours to make sure that we find these two missing people, if possible."

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Authorities said the cause of the accident remains under investigation. Crispen also dismissed early reports suggesting there had been a fire on board, confirming there was no evidence of one.

Television footage showed the boat almost completely submerged before disappearing beneath the surface.

Although conditions were sunny, local footage indicated strong currents in the bay, with choppy waters and white caps visible at the time of the incident.

The boat capsized close to Alcatraz Island, one of San Francisco's most famous tourist attractions.

The island was home to the notorious federal prison that operated between 1934 and 1963. Surrounded by cold water and powerful currents, it gained a reputation as one of the world's most escape-proof prisons and is now managed by the U.S. National Park Service as a historic landmark.

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