Patch it don't lose it.
Naaaah, you guys have got it all wrong. :duh
The ONLY way to go is get a knife pouch sewn onto your wetsuit leg, on the thigh. It is an oval piece of neoprene with a hole 3/4 the way up. The hole allows you to reach the handle of the knife, but the patch still covers the top of the knife, keeping it in place. Insert a cheap plastic knife sheath into the pocket to protect it from the blade. Easy peasy, never ever gets caught, easy to get knife out, safe as houses, and you won't ever go back:naughty
Of course, although this is a tried and proven perfect solution that most NZ spearo's use, dive shops will not encourage this though - it means they will sell far less knives.
I will try to attach an image of may mate with his...