Here's a little piece of advice for you if you're ever attacked by a shark: sharks absolutely hate it when you stab them with a knife that also shoots compressed air into them. It's just excruciatingly painful, see? All you need is, you know, a knife that has a CO2 container in the handle that's designed for blasting air into gaping wounds.
We've seen a design for just such a knife before, but now here's the real deal. It works on not just sharks, obviously, as I'm pretty sure any living creature would be put into a world of hurt by getting a hot air blast in a fresh knife wound. Apparently, they aren't selling it to civilians, just to the military, but as with most things there's probably a way to get your hands on one. Scary.