There is no best knife for all divers. Like most choices of equipment, what is best depends on the intended use. The primary use for a knife underwater is for clearing entanglements. If you dive in caves, the only thing you are likely to become entangled in is cave line which is essentially string. Anything will cut that. If you are diving in open water it becomes a little more complicated. What kinds of line are you likely to come into contact with in the area you will be diving? If you share the water with fishermen you may need to be able to cut fishing line, stainless downrigger line and/or fishing nets. Are you operating your own boat, or crewing a boat? If so, you may need a larger knife, capable of cutting anchor line and mooring lines. Are you diving shipwrecks? If so you will definately need to be able to deal with all kinds of fishing equipment as well as the lines, cable and debris that sinks along with a ship. A goog prybar or large knife might come in handy here. In some areas, knives are forbidden by the dive operators. In those areas, a pair of shears or a folding knife that you can carry out of sight may be desireable.