When I learnt to dive, large knives were de rigeur: perhaps it was the influence of 007 in Thunderball! I had a massive knife like a Roman gladius short sword attached to my calf. It could slice a baked bean tin in two, but I never once drew it in the water. When I went tech I was taught to attach a pair of shears on the belt of my harness to the right, and a z-knife to the left. The z-knife is just the thing for monofilament fishing line, and the shears will handle pretty much anything: I've seen them cut a penny in two. Knives are pretty bad with cables and monofilament, which are the 2 biggest hazards. Knives are nice to have around for poking things and as a tool. Unless yours if particularly large, I think your inflator is the best place to put it. With the knife attached, it will hang down, instead of float up, which will take some getting used to. I'd recommend a retainer made out of surgical tube: it's always a little disconcerting to not be able to find your inflator when you want it. The only reason for having a large knife is as a wreck diver, where its main uses are as a screwdriver (with a flat point) and jemmy, for prising out port holes. But you didn't hear me say that and I wouldn't do it myself...