The National Institute of Nutrition has just released their report that states you can eat just about as much as you want of "green" foods and not gain any appreciable weight.
So on the ship I concentrate on leafy vegetables,Granny Smith apples,chile rellenos, spumoni ice cream,anything made with pistacios (pistachio pudding,ummm), and margaritas.
I especially like to cruise over St. Patrick's day where I can basically drink all the beer I want....without worrying about gaining weight.
Furthermore, it is well known that things like broken cookies "leak" most of their calories and that food consumed on Saturday and Sunday are metabolized so fast that it is almost like not eating them.
Also, published reports have demonstrated that when you eat very chewy foods, eg. taffy, you actually burn more calories in the chewing process than you consume.
Add to that the new Coke drink that reportedly actually burns calories and you now actually lose weight by eating.
As with anything else, proper meal planning on board can put a kibosh on the urban legend that you gain weight on a cruise.