I know at least one Kydex/Concealex craftsman that is a white water kayaker. He uses a Spyderco in a Concealex sheath strapped to his PFD, or sometimes a fixed blade in another type of Concealex rig. Find the knife you like and then look into whether the sheath is appropriate for you.

My friend has been kayaking for at least 20 years, probably much longer. If I can get a hold of him (busy guy) I'll ask him what he is using right now.