Picture a knife that will inject 24g of compressed gas into a predator, expand its innards to the size of a basketball and freeze the organs around the piercing and kill the critter instantly. For divers this could be extremely useful when sharks start to get that look in their eyes and begin to edge each other into a frenzy. Once the first critter is inflated it will go bobbing instantly to the surface before it starts leaking blood, which (if all goes as advertised) will draw the remaining thugs...I mean, fish away from the diver.
The important thing is that regardless of the size of the critter that much gas would defeat its *swim bladder equilibrium or whatever it uses to achieve its operating depths and send it up, doubtlessly overwhelmed at the sudden loss of control and massive organ failure. And, the deeper the diver is when the gas is injected the more positive buoyancy and expansion will result, and as the critter rises unable to vent it may explode into irresistible, mouth watering chum, which is like a free shishkabob and beer buffet to its mates and other well adapted species that'll happily share in the windfall.
I've seen enough of those underwater films to know that more than one diver has watched as sharks and even stingrays (like the one who got a case of the ass and killed Steve Irwin) began to develop attitudes and then became unpredictable if not downright dangerous.
I can see where this tool would be a real comfort and could even the odds with critters like Moray eels, Barracuda, (and perhaps even a leopard seal in Antarctic waters) or even a dolphin if it suddenly becomes dangerously aggressive. (a check of federal laws reveals that if you kill a marine mammal you're basically fucked! So. make sure you turn yourself in otherwise the govt will have no way of identifying and prosecuting you)
It may not guarantee immortality, but it would be a great deal more reassuring than an ordinary diving knife or a spear gun in some situations.
The hardest part would be resisting the temptation of trying it out on some big, friendly grouper or sea bass that's hanging around while welders are working on an offshore rig.
I'm certainly sensible enough to resist that, but you youngsters who may find it just too tempting should consider the possible consequences of chumming your area and possibly attracting toothy company for the duration.
* Not all fishes have a swim bladder. Sharks for example do not have a swim bladder, and many species such as the Grey Nurse Shark, use a different strategy which includes having a large oily liver and specialised body shape to maintain buoyancy.
This knife is bound for greatness...and for infamy. Who wants to guess how long it will take before Crime Library has its first case involving one?
Poster Comment:
If one planned on using one on the street (during the Obama riots for instance) for two legged predators, it should be mentioned that the cops would figure it out PDQ and then contact the factory for the names of all who purchased one.