Diver Dennis
Contributor
I think I may start diving with a submersible machine gun - I know, I know, they don't exist.... yet!
Splash, Splash, You're Dead: The Military's Next-Gen Water Gun
Modern Firearms - APS underwater assault rifle
There is also a recently invented gun that fire projectiles with compressed air in them, which will obviously do some damage to humans as well as a shark. In real life, the chances of being hit by a shark are so low and if you were, you would probably not have any time to deploy and fire a weapon before you were chomped. I'm more worried about getting to the dive site alive through traffic than about anything to do with diving, especially sharks.