Not to bring you down, but I gained about 6 lbs on our trip....granted it was a three week Disney trip (1 week at the parks and 12 days on
DCL), so the odds were stacked up against me. I am a gym rat at home and tried to maintain that as much as possible on the ship, but I also love my sweets, which were very abundant on the trip. I tried to lift 2xs a week and do cardio (step, kickboxing) 2xs a week, in addition to snorkling and diving, taking the stairs EVERYWHERE on the ship (they are quicker than the elevators anyway) and making somewhat healthy choices as far as food goes....
But just to warn you...
Palo will kill you, but it is worth every yummy bite
Scoops is low fat, but certainly not low calorie, BUT SOOOO GOOD! (I ate there everyday for 12 days!)
Do your best, but realize you are on vacation. ENJOY yourself, enjoy a bit of everything, but not all of it. Try to fit workouts in, either before visiting a port, or early on sea days. Also, try the yoga on Serenity Bay. Even though it isn't much of a workout, it is quite a peaceful, beautiful thing to do!