To answer your question properly I would need to know what kind of diving, and where you were diving? It is never a bad idea to carry two knives, as for the size and shape of those knives it depends on what kind of diving you are doing. For instance, if you are spearfishing or underwater hunting in any way, you might want a longer knife with a pointed tip for dispatching of prey. It is also nice to hav a longer blunt tip knife if you do a lot of exploring and migt need to pry something. Personally I carry a blunt tip U.S. divers 4 1/2 inch blade knife on my inner calf, an Underwater Kinetics Remora BC knife on my BC strap, and because of the fact that where I dive there are a lot of old trot- lines laying arond I also carry a pair of SeaSnips ( EMT Shears ) as I have found them to be the absolute best for cutting any kind of fishing line or cord that me or might dive buddy might get tangled in. I hope this helps..
PS- The Remora BC knife by Underwater Kinetics is an awesome little knife and you can get them at Scuba Toys and lots of other places.:mean: