I'm doing weight watchers too (lost 70 lbs, got pregnant, am now 8 lbs over my lowest weight, so am back on now that baby is 12 weeks old). Here are my tips for doing great on the cruise, some weight watchers specific, some more general.
1) If you, like me, track electronically, nab a paper tracker before you go to keep track while you're on the cruise.
2) Remember that your goal does not have to be to lose weight on the cruise. You can decide ahead of time to either maintain or name an acceptable amount to gain.
3) On the up side, the serving sizes on cruises are very, very reasonable. So look forward to that!
Now on to the advice:
4) Pretend that the elevators do not exist. Take the stairs EVERYWHERE. My only excuse for taking the elevators are when i'm wearing heels.
5) Choose active shore excursions over passive. Kayaking instead of the glass bottom boat. Walking tour instead of bus tour.
6) Use the fitness area. Bonus pro-tip: The showers in the fitness area are way nicer than the one in your room, so go to the fitness center to work out followed by treating yourself with a nice long shower.
7) Buffets are tough. Here are some buffet tips:
a) Avoid the buffets if possible, they are almost always 'extra' food.
b) If you really want to do a buffet, do not get a plate right away. Walk the whole length of the buffet to see everything available first, make your decisions of what you are going to get.
c) If more than one size of plate is available, take the smallest size plate.
d) Take one trip. Do not go up for seconds. One plate of food is all you need.
e) If going up for dessert, take the smallest plate you can. I go to the coffee bar and take a saucer and use that for my dessert.
8) Use the fact that all your food is pre-paid to try new and interesting foods that you might not otherwise try. After all, you won't have to pay extra for them, and you won't go hungry if you don't like it.