One person has died and three others are missing after a pontoon boat capsized around 600 yards from Alcatraz Island on Tuesday.

San Francisco Fire Department received a report at 15:37 local time of a vessel in distress, department chief Dean Crispen said at a news conference.

A marine unit first on the scene witnessed a man in the water "in severe distress" and initiated CPR, Crispen said. He was declared deceased when brought to shore.

He said 20 people were on the three-deck pontoon boat, mostly family members, who were "engaging in some kind of memorial service".

Authorities said a search and rescue operation by the San Francisco Fire Department and the US coastguard would continue throughout the evening.

Three people on the boat were injured and transported to a local hospital, where Crispen said they were in a "stable condition".

The other 13 members on the boat returned safely to shore, and were not injured.

In an earlier news conference, the fire department chief said there were 19 people on the boat, and the report received at 15:37 reported a vessel on fire.

Later on Tuesday evening, he said witness accounts indicated there were 20 people on the boat, which was a 50ft cabin cruiser.

He said there was no evidence of a fire when the fire service arrived at the boat, with witnesses reporting rough seas and a vessel which began to take on water and was turned over in the bay.

Longshoreman Justin Marceline, who alongside fisherman Mike Montoya helped rescue people from the vessel, according to Reuters news agency, said it was "complete chaos when we showed up".

"Every time we had to grab a person, each one of them, either they were exhausted or they were frantic and were flailing," Marceline said.