JDS,
I've got about 15-16 years experience as a whitewater boater and what you will be dealing with most in a WW rescue situation is rope, webbing, nylon line, or possibly cutting out of a sprayskirt. The most efficient knife that also has built in safety considerations is the Spyderco Harpy or Merlin (now the Harpy ltwt). The knife was made originally for commercial fisherman cutting rope, line, and netting. The curved serrated blade gives a more efficient cut per stroke. If rope , webbing, etc. is wrapped around a the victim, the back of the curved blade can be placed against the body or limb, and the blade can safely be inserted underneath for the cut. A slip (you probably won't be on a stable platform) will only drive the point and edge up and away from the victim - safety. Clothing or equipment can be removed quickly and safely (in case of broken bones, etc.) using the same method. Carried in a Concealex or kydex sheath attached to your PFD, it's unobtrusive and easy to deploy in an emergency. I carry mine horizontally on the chest adjustment straps of my PFD and it stays totally out of the way 'til I need it. A couple of years ago an unconcious, upsidedown boater had to be cut from his sprayskirt after leaving the grab loop inside and was fataly injured (femoral artery) when the rescuers used a fixed blade to cut into the skirt (the legs will never be where they should - Murphy's Law). With a hawkbill blade, like the Harpy/Merlin, you can make a controlled cut with less than a 1/4 inch of blade entering the skirt - safety again. That's my recommendation based on my boating experience, knowledge of knives, and involvment in a few such situations.
Whatever knife you get, it should be serrated, stainless (that's not rustproof), and quick to deploy in an emergency. It should be of a size that will not interfere with your movements or snag on clothing or equipment and be attached to your PFD so you can access it quickly at all times.
The River Shorty is a decent river knife (there's a lot of them out there) but the factory sheath has a history of losing knives, as Copper found out. Advise you to get a replacement custom sheath if you get one.
I make sheaths for all sorts of knives, but got my start in the business when trying to find the best way of carrying an effective whitewater rescue tool for myself. Made a sheath for a Spyderco on a Rapidstyle vest (no lash tab), made some for my friends, then took them to Nantahala Outdoor Center, where they've been selling for several years. I'm not posting this to drum up business; your question just happened to hit where I live and I've been involved and concerned with River Safety for a long time.
SYOTR,
Mike Sastre
River City Sheaths