When I was a kid growing up in Monterey, I had a friend who's family were commercial fishermen. Believe it or not, the DFG used to supply them with something we called "seal bombs" which were basically M-80 explosives that were completely waterproof and would continue burning (and explode) under water. Apparently they were designed to scare off the furbags before they brought in their nets.
They also happened to work really well when flushed down the toilets of the high school gym......
Just found this link on legal deterrence of pinnipeds (knotheads, furbags, sea dogs)
http://www.nwr.noaa.gov/Marine-Mamma...-Pinnipeds.cfm
Apparently you can use forceful methods of deterrence as long as you don't:
1. break their skin
2. aim at their eyes or heads
3. disturb them from a position hauled out on rocks or beach
So, technically using a "seal bomb" would be legal to protect your catch or your gear, but I'll bet that possessing them is illegal for non-commercial fishermen (classic catch-22)