We went on a short cruise in the Med last year, the breakfasts and lunches were buffet style so no problem. The dinners were set menus, but the staff were quite happy to amend my main dish to a low carb serving, but only by cutting out the carby bits! I skipped the deserts (mostly) as they were all carby stuff.
I found I had to exert all my willpower to miss out on the delicious food on offer, and indeed did give in once or twice. It's difficult when everybody else is tucking in, even my fellow Type 2 passengers - of which there were a few!