I have a Kershaw 3.5 inch stainless combination serrated blade folder that I carry always, even on land. It has a Ken Onion-designed one-hand opening spring and peg, and a decent antislip plastic handle. About $70 at GI Joe's in Portland, OR. It's hard to keep sharp, as I end up using it to cut paper and line and food and soon-to-be-food. I keep it clipped inside the left pocket of my Mustang commercial PFD. I find that the clip tends to catch on my hand when I keep in in my jeans pocket, which is an annoying drawback that would make me reconsider my next purchase.
I have an old classic Leatherman tool that is on my belt even at weddings and funerals.