The ideal kayaking knife would be one that will not rust at all. For that you want Talonite, Stellite, Titanium, or H-1. Talonite and Stellite are the best of those, at least for most purposes, and they are also by far the most expensive.
My first choice would be one of the titanium versions of the
HideAway Knife (~$190). Titanium is completely impervious to rust and other corrosion, although it does not hold an edge quite as well as steel. The HideAway Knife is a really clever design that allows full use of the hand without any risk of dropping the knife. I am typing this while wearing mine.

It is also very quick and positive to draw and is available with a variety of carry options, including a Mini-TekLok. Expensive but well worth it!
If that's too expensive, my next choice would be the Benchmade 100SH20 River/Dive Knife (~$60). It is made of H-1, which will not hold an edge as well as a premium steel, but provides adequate performance and is completely immune to rusting. (Even the best "stainless" steel will rust.)
Another great option will soon be available, when Spyderco begins shipping their Salt model. It is a folder, very similar in design to the classic Delica, but with a partial-sheepfoot serrated blade, a larger opening hole, and all rust-proof fittings. (The blade and lock are H-1).
Whatever knife you select, you should consider getting a custom sheath made for it by Mike Sastre of
River City Sheaths. Mike is a whitewater veteran and he knows how to make a sheath that will be absolutely secure under the most extreme conditions.
--Bob Q