I think Carnival is the one that actually offers Atkins cruises. While you may not be taking one of those, contact the cruise line before your departure and inform them you are on a special diet. If you give them warning, most cruise lines are more then accomidating. After all making your trip the best possible means word of mouth and repeat customers.

There will be alot of temptations, constant buffets and dessert bars, but if you have SF desserts it may not be as bad.

You might also check out moving to CAD for a bit. That would mean no snacks and 2 meals very strict (more so then ATkins) but you could have a regular dinner. The no snacking would be hard though.

Most cruises also have exercise rooms, a pool maybe, or at least a ton of decks to walk. Get your hinnie in gear and burn off any extra carbs