We just got back from a nonDCL cruise in April. I also didn't want to gain weight like I had on previous cruises (usually 1 pound or more each day!!). Here's what I did to combat the buldge...
We were on the 7th deck of a 13 deck ship. Smack dab in the middle. DH and I never took the elevators (except on formal night-if I did stairs in those strappy sandals I would have broken something, LOL). I actually worked out--OK, just 2 times for an hour on the treadmill, but every little bit helps, right?
For food, we always ate at the buffet for breakfast and lunch. My main strategy was to fill my plate 2/3 with fruit and veggies/salad before I even looked at anything else. I stayed away from sauces, butter, etc. I tried to eat small portions of things that I absolutely love, and stayed away from things I make at home. I had only 1 drink (alchoholic) the entire cruise, and drank only water or unsweetened iced tea the rest of the time. I ate whatever I wanted for dinner. No late night snacks, except cc cookies and milk one night.
I felt so full all the time that I was sure I had packed on the pounds. I was so excited to find I only gained 1/2 pound in a week! I really think the stairs were a huge factor. That will be my strategy for every cruise from now on.
Good luck! And know it is possible to go and not gain weight--and not feel like you sacrificed anything!!