The Coast Guard received reports at about 8:30 a.m. that a privately owned 37-foot recreational boat had sunk and was leaking fuel near Coast Guard Island, Lt. Laura Williams said.
Responding investigators immediately saw the sheen and put up absorbent boom to prevent the fuel from spreading.
The owner of the boat, who was not injured, said there were up to 300 gallons of fuel in the boat.
Williams could not estimate how much fuel had leaked and did not know when the boat sank. She said investigators concluded, however, the sinking was recent because of the distance of the spilled fuel from the boat.
She said the area was not considered high risk to exposing oil to wildlife.
No injuries were reported as a result of the boat sinking.
The Coast Guard is working with the California Office of Spill Prevention and Response as well as the National Response Corporation.