all the other divers on the boat were carrying 6 inch Blades, and when we went down I discovered why , a forest of kelp waited below us!
This sounds more like a matter of local tradition then, not expert knowledge.
1) Rig your gear properly on the surface. Eliminate all the potential snag points you can.
- use vinyl tape to secure any dangling strap ends - mask, gauges, fins, and knife scabard, if you insist on wearing it on your leg.
- get rid of the snorkel, or carry it in a pocket if you will die without one.
- no dangling spgs or long tag ends on harness or weightbelt straps.
2) Treat kelp like a sem-overhead environment. Manage your gas supply accordingly and avoid nasty surprises.
3) A good buddy in invaluable for sorting out entanglements on your cylinder. Don't leave home without one.
4) Learn to move properly through kelp forests. Avoid quick, twisting movements. Be aware of any small tugs, stop and sort out the hangup while it's still one or two fronds, before you start hauling the entire plant with you. Trust your buddy to help you sort things out.
5) Kelp is patchy. Choose your route, rather than bushwhacking straight through the dense areas.
6) If you do need to "cut your way out", single strands of kelp snap by hand very easily. Multiple strands can be bitten through and are quite tasty, but I don't recommend this for your average tourist. 3" blades work fine if you cut intelligently (bundle up the offending stipes in one hand and cut the bundle with the other). If you need to hack and slash indiscriminately you should have been
If you do need to draw your knife, the leg is just about the worst place for it. To access it you have to twist yourself up in a figure "4", and the chances are very good that you will end up with more kelp twisted around your leg after you're retrieved the knife. A knife worn on the upper body can be retrieved without any entangling contortions.
In my experience, most knives are lost after they've been removed and used once then replaced in the scabard. Hard to see if the retaining mechanism has locked properly, I guess. The point, however, is that once you've drawn the knife you've significantly increased the risk it won't be there if you find yourself really needing it again. Better to avoid drawing it in the first place, or if you have to use it regularly carry it in a place where you can monitor its presence more easily than on the leg.
The only time I've seen a need for a large pig stabber was on a dive where the two of us were trying to stay on station on a gravel bar in a two knot current. My buddy was able to anchor in the gravel with his pig stabber with one hand and help stabilize me with the other while I wrote. If I wasn't being paid to write I would have been going with the flow.
