A dive knife has long been considered a necessary piece of dive gear for extraction in the unlikely event of entanglement. But more than that, the dive knife has become an important tool for light underwater work, exploration, probing, and hunting.
PURPOSES
Specifically what kind of dive knife you choose will depend largely on its intended use. If you are a spearfisher, you’ll need a razor sharp knife with a sharp point (for quickly dispatching a large fish, if necessary). If you like to take scallops, a sturdy, blunt tip knife is a must. But what any dive knife needs is the ability to extract the diver in a difficult entanglement situation. And what you need for that is a good, sharp edge.
EDGES
Dive knives come with a variety of edges for a variety of purposes. The most common is a simple knife edge. It should be razor sharp—meaning you should be able to shave with it—albeit not necessarily in a comfortable way, but it should take hair off your arm. This edge is for quickly cutting through line, kelp, or for dressing out game, although most dive knives are not especially suited to this task.
Saw or serrated edges allow for a more effective cutting of small branches or rope. Line cutters are odd notches in a blade intended to allow you to gather a handful of small lines, like fishing line, together and cut them all at once. They are also somewhat effective in cutting small rope. They can, however, reduce the overall strength of the blade.
BLADE MATERIAL
The metals used in dive knife making are an alloy of several metals and other elements, usually heat tempered and sometimes forged (hammered) to give the knife blade specific desired qualities. These chief qualities are strength (the ability to be take force without breaking or bending), the ability to hold and edge (a factor of metal hardness), and corrosion resistance. Less important qualities include cost and weight.