I've been on cruises before and I would bring an ice chest with tons of supplies and even a bag with pork rinds and whatever else you can think of that will keep you low carb when they're pushing the 24 hour pizza stand. Lunchtime for me was the worst because I couldn't find any protein or vegetables and I like both together.
Protein bars like adkins aren't the best but if it keeps me off of straight carbs and sugar, it's a great emergency fix. I'd rather eat 7-8g carbs instead of 30-40g. Slim Jims or any other meat sticks or Jerky with no added sugar. Nuts add up but if you can portion them to a certain amount, that helps as a filler when only garbage junk snacks are available like pretzels or chips.
Where are you going on your cruise, maybe you'll find some good low carb food off of the ship?
ps, If I were you I would bring a case of water because otherwise the ship charges $$ for each bottle and you won't drink enough.