Originally Posted by smurphny
While sailing, a whitewater PFD knife: immediately releasable, good grip, very sharp serrated, sturdy blade. Gerber makes a plastic handle knife for around $40. I have an older all metal model. IMO, anything you need to unfold will likely stay folded if needed in a hurry. Just imagine trying to find and unfold a knife as that anchor rode is pulling you over. Commercial fishermen often use an ankle sheath in case they get hauled over the side, tangled in gear. It is quicker to find the handle on your ankle. If being dragged under, you have only a couple of seconds before being dragged too far under for it to make any difference as far as the outcome. Those "Leatherman" knives are very useful. I have one with at least a hundred applications but it is useless as an emergency knife.
I'm not sure any knife is good for cutting yourself free from a tether. Nothing i see above is going to be opened while you are being drageed behind a boat. For cutting the tether, I have a razar sharp S-cutter on a lanyard inside the PFD. The blade is covered in wax, which would slice through when pulled against the line. Nothing to open and it sits inches from the tether itself.
You must be referring to the Letherman multi-tool, which I agree is not good for emergencies. However, the Leaherman knife is great.