This past summer I bought a big, sharp, shiney, titanium, knife with a clear hard-plastic sheath at a pawn shop for 20 bucks. I thought this was the bargain of the year for me. The thing was a "rambo" for sure! Heavy and about 12" total length. What a useless piece of junk! I had no straps with it so mistakenly bought some nylon straps with buckle harnesses. Of course when diving 100', wetsuit compresses and sheath straps don't! I messed around with that stupid thing for two dives and on the third, the sheath became loose completly and the knife became released and dropped out. I didn't realize it until I was on surface, but was not concerned one bit. Keep an eye out for it on the Lilly Parsons, Brockville, St. Lawrence River. I don't want it back, use it in the garage for opening paint cans or something. There is lots of good information here about knives, and I'm glad for the info.
One point we should all keep in mind is the Buddy, redundancy thing. I know where his knife is and he knows where mine is. I can cut him out and he can cut me out. Much safer that way I would think. Rather than trying to cut yourself out of a big mess?