Steven Hammond and Stephen Cuss were walking along a pier in Aquatic Park when they spotted the man in the water.
“He was face down in the water with a life jacket on, and we thought he was going to go under, so Steve went under the pier and got him started, and got him up the pier,” Hammond said.
Hammond is a lifeguard in the U.K. but says they were not able to revive the man before paramedics arrived and transported him by jet ski to an ambulance at the beach.
The man was taken to San Francisco General Hospital, where he was listed in critical condition.
It was unclear why the man was wearing the life jacket, or where he entered the water.
This is the third major bay rescue since Thursday.
Fire crews have been gearing up for the holiday weekend, as many residents and visitors are expected to head to the beach.
In preparation for Labor Day on Monday, crews trained Sunday morning at Ocean Beach, fire department spokesman Lt. Jonathan Baxter said.
Fire officials are asking beachgoers to be cautious of dangerous conditions such as strong rip currents, large shore breaks and cold water temperatures, which could lead to hypothermia.
Additionally, the fire department recommends that beachgoers head to beaches where trained lifeguards are stationed, such as Stinson Beach in Marin County, Baxter said.