Like everyone said, depends on where you want it, what its going to be used for and how you feel about it.
I have a basic nice line cutter on my pfd, I figure thats all I need for that, benchmade makes some really nice ones, and if you can find an older one, they use to make ones for water use! very safe!!
I also carry a folder in my pocket at all times (every day as well) If you are looking for a aqua knife, from experience Spyderco makes the best of the best for work/rescue knives specifically designed for rough water use. Other people make good knives, but when it comes down to it, for use, function and dependability... spyderco!
I also have a fixed blade on my pack that sits in my canoe, its a CRKT abc diver, dive knives make GREAT knives to attach to a pack or on your boat if you are a yaker (witch i remember you being

) I canoe, so i have a pack in front of me most of the time, and its just tied to that.
Things to remember or think about... Do you need a point, or want a blunt nose knife, do you want a folder or a fixed blade. Where do you want it, do you need one or two,
make sure its made for water use as well, non water use knives are harder to clean and keep functioning after they have seen the river or lake!
and most of all, a sharp knife is a safe knife, keep it Sharp at all times!
I collect pocket knives and would love to show them off if you would like to play with some! I could try to make it to a meeting some time or meet up some where if you like!
Good luck in your quest!
Bryan