For any of us who have spent enough time on the water, have seen swimmers in distress, it's clear that when things go wrong, they will go wrong in a hurry.  Especially in low light situations when serious environmental factors are present.

Coming from an extremely competent swimmer, (just not a good speller ;0) I would never tether a kayak to myself, no matter what quick release system I was using or whatever hook-style knife that I was carrying.

Once you launch, if you are not prepared to dump and break free and clear from your yak at any time...and survive, you don't belong on the water in that situation.  The paddle leash is the extent that I would ever go.  The money invested on everything sitting on your yak, is nothing compared to the value of your well being.

The paddle leash is the extent that I would ever go for a tether.  For those of us wearing waders, have we tested them to see what kind of swimming performance we will get when they fill up with 48 degree water?  Ever practice ditching them under ideal conditions?

More thoughts: a signal device such as a strobe light and some sort of water-proof vhf radio on my person for night fishing would be ideal safety equipment.  For daylight situations, add a pencil flare and you're more than covered. 

Of course making sure you always go out with a competent fellow yaker is your best insurance policy.