If you want to get into an underwater knife fight or are a spear-fisherman, then what you need is a leg knife that comes off as you enter the water
If you want to look like you take SCUBA diving seriously, all you need is a minimalistic line cutter with next to no drag. This line cutter will cut through webbing and BCD harnesses like a hot knife through butter, whilst keeping the diver safe at all times. The harness attaches through a sewn on strap (meaning it should never come loose) and the knife attaches into that through Velcro. The blade will last a very long time if looked after, and if you don't look after it, no problem - it comes with a spare.