A boat carrying around 20 people sank in San Francisco Bay Tuesday afternoon, and at least one person is dead and three others are missing.

An emergency rescue took place Tuesday afternoon just after 3:30 pm near Alcatraz Island, where cabin cruiser-type boat became submerged in the water.* As the Chronicle reports, 16 people were reported rescued, and the Coast Guard was transporting survivors to GasHouse Cove Marina, near the Marina Safeway and Fort Mason in SF.

Authorities confirmed that the boat was a three-story, 50-foot cabin cruiser named Volare, which launched from the San Francisco Marina. The passengers had reportedly gathered for a memorial and were mostly family members.

While the cause of the accident is still under investigation, San Francisco Fire Chief Dean Crispen told the Chronicle the boat capsized after it was struck by a wave. Video from the scene showed possibly blustery or rough-water conditions.

ABC 7 reported seeing one body covered in a tarp at the marina.

According to the New York Times, by the time rescuers arrived, the boat was almost fully submerged.

“Some people were in the water,” Chief Crispen told the Times. “Some people were on the boat. Some people were just falling into the water.”

Crispen said the person who died was pulled out of the water alive with severe injuries, and rescue workers attempted cardiopulmonary resuscitation.

KTVU reported that two people remained unaccounted for, but authorities later updated the number of missing people to three — at least two of them women. Thirteen people were seen onshore and three others taken to the hospital. A dog also reportedly died in the incident.

As many as nine of those rescued were reportedly injured, per the Chronicle, and 11 rescue boats were reportedly involved in the rescue effort.

The incident happened around 600 yards from the shore of Alcatraz, in water that is reportedly 120 feet deep.

Boating expert Steve Jones, who has decades of experience, told the Chronicle that pontoon boats are not designed for the exposed, often rough waters around Alcatraz and are better suited for lakes and protected waterways.

“Almost anybody in the business would tell you they’re not safe on the bay,” Jones said. “They’re not designed for it.”

It remains unclear whether the boat was privately or commercially operated.

Some initial reports suggested there had been a fire onboard the vessel, and while the reason for the sinking remains unclear, the Chronicle subsequently received confirmation from "officials" that an explosion had occurred onboard the boat, and then later rescinded that detail.

Related: Owner Had Taken Capsized Boat on Open Ocean to San Diego

*This post has been updated and corrected throughout. Initial reports suggested the boat, given its partially submerged appearance, was a flat or pontoon-style boat, but it was actually a three-deck trawler or cabin cruiser with a sunshade over its flat upper deck.