A knife that injects compressed gas into the victim

 

Designed for underwater use:

This weapon injects a freezing cold ball of compressed gas, approximately the size of a basketball, at 800psi nearly instantly. The effects of this injection will drop many of the world's largest land predators. The effects of the compressed gas not only cause over-inflation during ascent when used underwater, but also freezes all tissues and organs surrounding the point of injection on land or at sea. When used underwater, the injected gas carries the predator to the surface BEFORE blood is released into the water. Thus giving the diver added protection by diverting other potential predators to the surface. 

More info here, video here