I enjoy cruise vacations, the variety of different places with consistently good service and comfort.
I sincerely doubt that any SP's go out and book themselves on cruises, with the idea of selling sessions. The cruise will only have 1000-3000 people on board, many of whom are there with families, wives etc. I just can't see it being economically viable for the lady. You can always sample the local talent in your ports of call, just make sure you are back on the boat before departure time! In Europe, that's likely your best bet. There are also single events on most cruise lines and you might get lucky with a hook-up or two. Sometimes sisters or friends will take a cruise together, or if you like the GILF's, you can check out the older ladies. Good luck scoring with the crew - it's very very rare from my experience, although some of them are smoking hot, especially the entertainment and casino staff. Very few will risk their job by screwing a guest. You can try, but the ship is not that big, you would get a rep among the crew pretty fast.
Another option would be to take someone with you, if you can work out a decent rate. That topic has been beat to death here and my views on it are well known.