Don't let a cruise ruin your diet

I've never been on a cruise, but I imagine going to one would be like visiting an all-inclusive resort, where the buffets and bars are always well-stocked and the entertainment flows just as much as a booze. One thing I know about all-inclusive resorts is that dieting, or even maintaining your weight is next to impossible. At least, if you're will-power-less like I am.

The reason I'm talking cruises is Fitsugar has posted some suggestions on how to keep from de-railing your diet while on a cruise, like:

• Eat 5 to 6 small meals a day, instead of 3 huge ones
• At a buffet, fill your plate with healthy things like salad first and hopefully there won't be room for the unhealthy stuff by the time you get to it.
  
• Drink lots of water
• Sign up for activities, especially the fitness classes.

How do you stay fit on a cruise?