This guy is a friend of a friend, you should check this out!
http://www.californiapaddle.com/
He is using an electrical field of some kind on his board that apparently deters the smaller predators, sharks, seals etc.,and other nuisances. When he finishes his Trek from Canada to the Mexican border I will ask him more about it. Something like that might work, a taser, cattle prod, or even an electrical field designed to dissuade seals, and other predators. I think it only works within a few feet of his board though...hmmm....
Anyone want to meet up and paddle with him through La Jolla?