I have played quite a few contracts onboard various lines. Carnival is the best. The most important thing that I have heard here is the "be careful with your chops" thing. If you go out there, make sure that you maintain a very good warm up daily. I have seen and known players that have gone out there and ruined their chops. They would stay in the "crew bar" drinking .50 cent beers (yes, .50 cents!) and then sleep in. They would sleep in until 5 minutes prior to the gig or the rehearsal with no time to warm up. Guys would sit there, all hung over trying to make that E above high C. It was painful to watch.....and to listen.
Other than that, easy gig. The money on the ships is too low however. The pay is actually quite insulting for the amount of playing involved.
Good luck.