Officials said the animals, from the U.S. Navy Marine Mammal Program, found the dummy mine on a pier piling near San Francisco's AT&T Park as part of the three-day statewide exercise, which focuses on port terrorism, the San Francisco Chronicle reported Wednesday.
The sea lions and dolphin team also spotted two suspicious divers and apprehended one of them with a leg cuff.
"They inspired me," San Francisco Mayor Gavin Newsom said of the exercise. "It is a reminder that we're not the United States of San Francisco. We are part of this country and there are those that wish to harm this country. San Francisco is on that list. We can deny it all we want, but we will deny it at our peril. We need to be vigilant."
Mark Berman of the Berkeley, Calif., environmental group Earth Island Institute objected to the use of the marine animals in the exercise. He said it is ethically wrong to use marine mammals to perform tasks for humans.
"Dolphins deserve to be in the wild on their own terms," Berman said.
2014: The Year in Music [PHOTOS]