A standard scuba diver tool, primarily because, strapped to your leg, you just look so much more 007-style lethal. Also, they are quite useful for cutting away lines and entanglements, as as well as chipping, prodding, prying - just about any need or emergency that should arise underwater.

In reality, diving knives are tools, not weapons. Get the blunt tip version so you don't poke a hole in anything (or anyone). The Ti6 is one of the best diving knives I've found - made from Titanium, so it practically never corrodes. The holster and leg straps are top notch as well, as can be expected from Atomic products.