From stories like Lee's of cat sailors fortunate enough to escape a lung-full of water or worse, the best guidance was to have a non-folding knife easily reachable by either hand. If you only have one hand available, a folder is going to be a problem. Also, recognize that it is highly likely you will have less than a full lung of air when you go under...so you can't count on the 60 sec or so that you can normally hold your breath.
One story I was told (firsthand) by a guy getting trapped unther tramp of an I20...he had to cut through the tramp to reach the surface. He will now only carry a non-folding knife with at least a 4 inch serated blade.

I have a titanium River knife affixed to my pfd horizontally along the waist. I can access with either hand and it is out of the way when not needed.



Mike Dobbs
Tornado CAN 99 "Full Tilt"