ID, a machete will replace most of that stuff and really make people respect you...
Since I am not Richard Widmark (You mean all you guys did not see his movie about frogmen? In black and white!) so I am not comfortable strapping a knife to my leg. I went through a lot of options, though. The highlights...
Started with a BC knife because we do not generally dive in lots of kelp, etc. and did not think I needed a Crocodile Dundee Special.
The first place I put it was hanging upside-down on my a D-ring on my BC--armpit level. That was okay, but I wanted to use the non-business end as a tank banger. So I used "Technical Black" wire ties to put it on my consol HP tubing; it was convinient, visible and I could reach my tank for banging. However, I damn near lost the thing when our BC's were being set up for our second dive of the day. Somehow, someone managed to push the keeper in, releasing the knife.
So, now it resides in a pocket on the end of a retractable line. That keeps it safe, out of the way and usable.
One interesting comment...my first small knife was a McNett BC knife, supposedly made from 301 SS. First trip out it rusted badly--even though I rinsed it when we got ashore after each dive(s). I took it back to the dive shop and they told me all knives corrode and that no refund was in order. I do not go back to that shop and they have "lost" thousands of dollars of business, mine and my friends--over a $25 knife. I do not think that 301 SS should corrode that way and people I know who have a similar size and shape knife called "Remora" do not have the problem.
By the way, I took the sorry McNett home, buffed out the rust with the appropriate wheel and compound, and coated the blade with a clear plastic "paint". It held up a little better, but would come back corroded from most trips--I know, I know, paint is porous and H2O and NaCl are "small" molecules. So, back to the buffing wheel. I then coated the newly shiny--with little pits here and there--knife with silicone oil. That worked, but I had to recoat it regularly and that was a pain. Bottom line: to use the words of my sons, McNett sucks (blush, blush).
Joewr