With so many choices on the market, how do you decide which diving knife is right for you? It really depends on the type of diving you'll be doing and your price range. Dive knives come in a variety of shapes and sizes. A dive knife should: have a sharp cutting edge and serrated sawing edge, come with a sheath or holder, and be made of stainless steel or titanium.
Blade Size and Tip
For most recreational divers, the best choice is a medium length blunt tip. An example is a knife with a 4-inch blade and a 4 to 5-inch handle. A dive knife this size is easy to grip, can be mounted to your BC, and there is less risk of puncturing your equipment, your dive buddy, or yourself. If you like to spearfish, choose a sharp-pointed knife. You'll need it to cut the fish.
Dive knives come with straight edges, serrated edges, or both. Serrated Edge: good for cutting natural lines, ropes, nets or kelp. Wavy or rounded serrations perform better than large saw teeth. The teeth should run the length of the blade. Plain Edge: good for severing small lines such as fishing line. A plain edge with a bit of a curve is preferred over a straight edge.
Mounting Options and Sheathing
Decide where and how you want to mount your knife. Choose a dive knife that offers the best choice for doing so. Knives that clip to your BC, hose or equipment are favored over leg-mounted knives. You want your knife to be easily accessed with either hand and easily removed. Look for sheathing that can be removed and replaced smoothly. Also, look for one finger quick releases.
Most dive knives are made from stainless steel (306, 420 or 440 alloys). The higher the alloy number, the less corrosion resistant. However, higher alloys do hold a sharpened edge longer. A more expensive, but higher quality alternative is a titanium knife. Titanium retains an edge longer, is rust proof and is lighter than stainless steel.
Diving knives are practical tools used by divers to cut, measure, pry, dig or pound underwater. A dive knife is NOT a weapon. (Don't use your knife to hunt or wrestle sharks.) If you are taking a diving knife on a trip, do not pack it in your carry-on bag. Pack it securely in it's holder in your checked luggage. Contact the airline you intend to fly with to find out their baggage policies.
Special thanks to About.com for this information