1. Diet before you go. Much easier than trying to diet after being spoiled on the cruise.
2. Build as much exercise as you can into your days. If possible, use stairs on the ship, and walk at a good pace. Lets you pack more fun in if you get to things faster!
3. Choose fish and veggies whenever you can at meals. Got my DH hooked on fish thanks to cruising.
4. Eat a protein loaded breakfast, very light lunch. Not good advice in general, but those cruise dinners are too good to miss, so I prefer to
"save up" for dinner. Skip late night buffets. Yes, even the chocolate one!
That's my approach, anyway, except for the stairs, which I can't do anymore. I have health and increasing weight issues.
Cut yourself some slack. It is a vacation!