I'd say if you're thinking of strapping a knife to your leg... Just don't. It's the worst place in my experience. It is not streamlined, is an extra step when donning or doffing your exposure protection, the curvature of your leg muscles often causes the catches to bend and unlock your knife, the movement of your legs can work the knife and straps loose, and it creates a place for line to catch on your legs where otherwise it is likely to just slide off. This goes for spearfishing as well as scuba. Look at professional spearfisherman such as Daniel Mann. If leg mounting was advantageous he'd do it. Mounting on a waist strap (or anywhere except for a limb) is a better option! A trilobite or j-cutter is awesome for the vast majority of uses. I'd highly recommend them. They're cheap, extremely easy to maintain (mine doesn't seem to rust at all), super easy and safe to use, lightweight, small and easy to mount, etc. I'd say this is the best default cutter that can safely and effectively be used by anyone. Many (most?) recreational divers don't seem to carry any cutting device which is really silly given how important it is to be able to free yourself of fishing line or rope. Ideally you should actually have 2. I opted for EMS shears as my alternate because they have the ability to cut through thin wire (like heavy fishing tracers) better than a knife. Though a knife is a fair choice too. A common choice amongst DIR/technical divers is a modified serrated knife with a plastic handle (like a cheap steak knife). They cut the blade down to just a couple of inches and round off the end so that it doesn't puncture skin or drysuits. You can find pre-made ones with sheaths online.
It's a good idea to carry multiple knives. Either duplicates in different locations that you can reach with both hands, should one get tangled up, or different knives for different purposes. The knife/shears combo is really bad for cutting through a BCD or harness, or cutting lines/ropes from animals, since there are sharp edges on both sides of the shears that will cut the animal or person you're using it on. Best to have one or more line cutters, since they're the most versatile. In addition, a one-sided serrated blade (like the Spyderco) can be useful when you need to cut through thicker rope. Another excellent option is a set of trauma shears. They're designed to cut through fabric without harming the person wearing it. They're cheap, but they're usually also rust prone. So take good care of it by rinsing and drying it thoroughly after a dive.
I got a line-cutter and a potato-cutter like knife on my settup. Many of my buddies (and I myself) use Wings. Usually without quick-release bottons, or no strap-opening at all. In Emergency, fastest and saftest to simply cut them of their/my wing (back at the beatch of course ^^). The backup simply for redundancy issue (came free with the wing). Nice if rope won't fit inside the cutter. Most important use is to hook my longhose behind for streamlining purpose.
True? Wow, such good memories for me when I was a kid. And I like it with butter and garlic roasted on a fire. Or cut into strips in chowder. Thanks for the update. Freddy
Great choice if it fits properly. Remember, even the highest rated mask in the world isn't worth a nickel if it doesn't fit. Thanks for watching. Freddy
When I was too young to afford this hobby, I believe the common wisdom was "you want a single smooth edge, macroserrations offer nothing microserrations don't, and don't get a reverse edge because that'll just complicate the balance of fitting it between the line and skin without cutting someone. Keep it simple, fixed blade is the only reliable choice." Now, trying to find a single-edge fixed blade with a blunt tip seems impossible.
We sell them on our website. www.Leviathanscuba.com. Remember that if you dive a protected marine park, they will not allow a knife period. But you can more easily hide a foldable dive knife. Just thinking out loud. Thanks for watching. Freddy