Dive Knives

There are 3 reasons why you might need a dive knife:

You get tangled in kelp or sea weed
You get caught in a fishing line or underwater fishing net
You are an international spy being attacked by enemies and need to cut their air hoses in a dramatic underwater fight.
Get a knife with a sharp, serrated edge. The serrated edge stays sharp longer and will let you saw through tough materials.
The knife should not have a pointed tip. A dive knife is never used to "stab" - in fact many dive knives have a screwdriver-like tip for wedging and prying.
A handle with a hard butt is useful for hammering
Look for a big, safe hilt guard between the handle and blade to prevent your hand ever slipping over the blade while using the knife
The knife clicks securely into its sheath, can't be shaken out accidentally and the sheath straps onto the outside of your thigh.
A magnetised blade can pick up little wee metal things from the sea floor.