I have always carried at least 2 cutting devices and usually 3 all placed in different areas. depending on the rig it can be hard to find a good place but something all BCD's have in common is a hose for adding air.
I use Zip ties (at least 3) to tie the knife upside down to the ribbed hose and this has been a common place from what I have seen for others to put their knife.
other places I have seen include waist straps, in/on ds/bc pockets, right shoulder strap, on a reg/hp hose, attached to the side of your canister light head, or any other light, attached to a pony/stage/deco bottle(slung), sideways on a chest strap, attached to your reel, attached to a helmet, clipped directly to a D-ring, arm/wrist mounted, on the back of a console, on a weight harness shoulder strap, on the bottom of a backplate horizontally, etc.
Although many of those options would make replacing or even finding the knife difficult IMHO that doesn't stop people from getting creative. I would suggest whatever you decide to get your gear on in a pool or something with a buddy to assist and make sure you can remove and replace the tool without damaging gear or hurting yourself with your thickest gloves on. Being able to reach the tool with both hands is also important especially if its your only knife.
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