Yes, I carry a knife in a sheath on my right leg. If there are entanglement hazards, it goes on the inside of my calf; if not, it's on the outside (easier to reach). What have I used it for over the last 40 or so years? Well, here's a sampling:
--Cut a net off a ships prop, which included cutting through a 4 inch rope that was holding the net. This was a mission ship, and our teenage dive club was in the area, so two of us cut it off after making sure that no one would start the engine.
--Cut parachute cord off a dead pilot who had been ejected through his canopy when his T-33 crashed into the Yellow Sea during my pararescue days.
--Cut through monofiliment line on many of my current river dives. I sometimes collect the fishing lures left in these masses of line on the bottom debries.
--One time I collected a live fish that had been caught, but was tangled around a root. The fisherman had to cut his line, and I came by some hours later and caught the fish that was tied to the stump's root. It was a nice 8" rainbow trout, and in my family I was the only one who caught a fish that day, and I wasn't even fishing.
I like a blade which is about 6 inches long, has a serrated edge as well as a knife edge (I used the serrated edge on the rope on the prop, and it works much better than the knife edge in that application), and is very sharp (which makes it easier to cut things). A sheers won't do some of the things a good knife will do. Mine is a Wenoka with the Z-lock system. I also like a bright handle (so I can see it if I put it down somewhere), and a one-handed locking system.
SeaRat