Well, assuming that you're on any kind of sailboat big enough to be doing this, you won't have need of a multitool, because you'll have real tools readily available to do that kind of work.
I'm going with the crowd here and saying that you'll want a Salt series folder for your every day knife. Get it in yellow, easier to find if dropped.
For the fixed blade, the Aqua Salt is the same kind of steel, which is a very good thing. Be sure to also get a sharpener (the Spyderco sharpmaker is excellent.)
What kind of boat do you have? A couple of friends of mine went around in a Westsail 32 about 10 years ago. I also have dreams of blue seas and white sand, but for now I'm stuck in the prairies.