Low tide occurred at about 7 a.m. today and tipped the barge over, causing it to start leaking, Capt. Mary Gutierrez said.
A kayaker reported the leak to the U.S. Coast Guard, which notified San Jose fire, she said.
The hazardous materials team placed absorbent pads in the water to soak up the spill, but the Coast Guard was still working this morning to figure out how to right the boat, Gutierrez said.
It was not clear how much fuel had leaked into the water.
The leak was mostly contained, though, Gutierrez said late this morning.
The hazardous materials team was still responding to the spill as of about 11:30 a.m., along with the Coast Guard and the state Department of Fish and Game.