You have to have a bangstick for sharks. Simple things, just poke the shark and a 12 gauge shotgun round fires. It either chases them away or pisses them off for a quick kill. :)
Spear guns are for edible stuff that can't make you dead in a big hurry.
Would a torpedo work?
Absolutely. But you might have a small problem getting clear. Water doesn't compress, but the human body does. In an underwater explosion it would be problematic.