Everyone will have their personal preferences, mine is a fixed blade with a blunt tip which can be attached to the shoulder strap for one handed withdrawal and use by the dominate hand. The downfall is having an exposed blade which can sometimes be difficult to control in a distress situation. A few situations which call for a good knife include cutting yourself free in case of entrapment, detaching from an anchor, cutting free from tangled line, cutting the leader to release a fish we don't want to handle or is taking us someplace we don't want to go, and of course cutting bait and filleting fish! (I'd suggest a separate knife from the rescue knife for bait and filleting!)
From a rescue perspective the safest knifes are the rescue line cutting knifes with a protected blade, they're ideal for cutting free from an anchor line, cutting lines causing entrapment, cutting leaders, etc. (Chad, are you still using one?)
Whatever the decision, keep it cleaned, sharp, and oiled to prevent corrosion / rust.
Vic 

I like the rescue hook idea that Cory suggested.  See below. I keep one on my left shoulder strap. If all I'm doing is cutting anchor or fishing line or my feet out of a mess, this should do it and I can't stab or cut myself.

I use the Benchmade Rescue Hook for line cutting and the Gerber  RIver Shorty blunt tip sheath knife for utility purposes.

Chad

The NRS Pilot, available from Appomatox River Company or WRO is my new kayak knife. It's like my dive knife, in that it has a blunt tip (screwdrive or plying tool, if necessary), attaches to my PFD lash tab, is a fixed blade and can be accessed with one hand. It has a built in line cutter, serrated and smooth blade edges and a bottle opener! Since it's chrome, it can even double as a signal mirror in a pinch! It also allows me to leave my dive knife with my dive gear. However, I highly recommend some WD-40 on the blade and a good fresh water rinse after each use.