Seal bomb - basically an M-80 firecracker in a red, round waterproof hard plastic shell, with a waterproof fuse. Makes a BIG boom! Generally, not always, scares the sea lions away from the boat, but doesn't physically hurt them or blow them up or anything.
As far as scaring the fish - not on the deeper reefs. The fish are a couple hundred feet down and sound (knowing this from my Navy anti-submarine warfare days) bounces off the thermocline (area of colder water under the warmer water on top) and reflects back up to the surface.