After going through the "5 inch blade knife strapped to my calf" period, during which I did some kelp diving, I settled in long ago to a 3 inch blunt end knife that is retained snugly in a sheath that mounts on my bcd and is easily accessible and returnable. I have one on each the bcds I use regularly (3). It has proven adequate to cut a line, to dig in the mud, and to bang on my tank to get another diver's attention. It can cut a sausage, spread peanut butter on a sandwich, cut up an apple, and anything else I need a knife for during or between dives. Since you don't have your own bcd, you will be glad to know that knives this size will come with forearm or calf mount straps. Get a dive knife, not some other tool not designed for underwater, especially under saltwater, use. It will last a long time and in the end your save money with quality, long lasting equipment.
DivemasterDennis