The man was in an 18-foot vessel with another individual when the boat began taking water and capsized near Potato Slough, west of Stockton.
The elderly man, who witnesses said was not able to swim, hung on to the hull of the vessel for about five minutes until a passerby rescued the pair, Coast Guard spokesman Lt. Andrew Kennedy said.
The Coast Guard received a call at 12:07 p.m. about a capsized boat with debris in the water, including food and coolers, Kennedy said.
It was unclear what had happened to the passengers, so the Coast Guard prepared for a worst-case scenario of people trapped under the hull. They launched a 25-foot response boat from Station Rio Vista and an HH-65 Dolphin helicopter from Air Station San Francisco, Kennedy said.
He said it turned out the passengers had already been taken to shore and did not need medical attention.
"His life jacket saved him," Kennedy said of the 82-year-old man. "This shows how important it is to have a life jacket for each individual on board."
Federal law requires children under 13 to wear a life jacket on boats at all times.