As with a lot of things, a dive knife is something I'd rather have and not need opposed to need and not have. Though as an advanced diver and Instructor I'd rather have two, in case I can't reach one or lose one - but where to keep them?
SCUBA equipment manufacturers offer a huge range of dive knives with different attachment options, but which would be suited for your way of diving?
Most commonly found on the market, are the leg mounted knives which attach to your lower leg above your ankle and can be a nice place to keep your knife out of the way. However sometimes this can cause more problems than its worth, having your knife that far away from your hands can mean that it is hard to find or even reach in an entanglement. Stretching to put a pointed knife back into its housing can also be dangerous if you miss the housing or do not engage the locking mechanism properly. Mounting your knife on your leg can make it a snag hazard, I find it quite easy to get just my fin buckle snagged on a line let alone a large dive knife.
More and more BCD's are being produced with knife grommets and dedicated areas to mount your knife, typically under your left arm above your pocket. These are great as you leave the knife attached to your BCD so you have it for every dive and can never leave it in the bottom of your box. These mounts are discreet and offer little chance of snagging but again you must be careful when replacing the knife after use to make sure you do not puncture the bladder of your BCD or yourself. This mount has another problem, if you are like me you can easily forget to wash and dry your knife after use and find it quite rusty when you come to use it next.
Nowadays I'm seeing a lot of divers mounting their dive knives on their right shoulder strap or on their BCD inflator hoses, these are nice and close to hand in an emergency but I'm not a huge fan of having something sharp that close to a regulator hose. Having your knife on your upper body increases the likelihood of you being able to reach it in an emergency and put it back safely. Personally I keep a cutting hook over my right shoulder and a small blunt-tip in my drysuit / BCD pocket.
What are your thoughts? Where do you keep your dive knife?