Lots o' people say they gain weight on cruises... but really, they have AWESOME food options. For breakfast, they have giant fruit bars along with sooo much other healthy stuff. I think it's all just keeping in mind what you're trying to do and make sure to eat as healthy as possible. Of course it's okay to treat yourself... sometimes... because they've got lots of good stuff, but, just try and keep it in moderation. They most likely have a gym, and if not I'm sure that you'll be keeping plenty busy, anyways!
I hope you have a TON of fun, and let us know how it went! You can do this!