I don't usually carry weapons on the boat, but if you carry some diving gear seems like it would be reasonable for you to also have a Bang Stick. Available in calibers from .22 to .357 and shotgun shell sizes. Because it is a point blank weapon (as they get close enough to identify as sharks or gators) they are effective even when firing a blank. Firing them out of the water voids the warranty of course.
Bang Stick - Mahalo