I prefer small and sharp, and as snake alluded to, cheap (yep, i have lost a few over the years.) I keep a small z-knife attached to the harness webbing on my right shoulder, as well as a small folding lock blade knife (Gerber River Runner) on my left waist belt (I like them on opposite sides and locations to ensure that I can always reach one of them.) I do have a BA dive knife from my specops days, but all it ever gets used for these days is to make and/or pund tent stakes into the ground. Whenever I venture out on a wreck (or just going out spear fishing) I will usually put a pair of shears (must be strong enough to cut through steel leaders) in my pocket.