I usually find on cruises that the breakfast and lunch has buffet options, and the choice is massive, so I have eggs from the omelette station, or smoked salmon and cottage cheese, or rollmops, and some slices of melon. (oK, i’m not that much of a saint, I have some sort of pastry as well, but there have always been mini sized croissants. Or I have a slice of pumpernickel or rye bread)
Then I have some form of protein with salad for lunch, and more fruit (melon or strawberries have always been available.
Evening meals in the restaurant have always consisted of a starter that’s fairly low carb, a soup course (that I normally skip because I just can’t do four courses) a main course option based on a lump of lean protein, loads of veg, and some form of potato, of which I just eat a little bit. Puddings tend to be light, and small portions.
Added to that, we normally walk a lot when we’re in port, and quite often don’t get back to the boat for afternoon tea, so I may be the only person who's ever lost weight on a cruise! But if we do make it back, again, cakes are portioned into small pieces, for you to have one piece (or six) as you wish.
There's always the option to have vast amounts of food if you want, (nobody counts how much cake you’re taking) and I have seen people stuffing their faces, but I think cruise lines are ever aware of their proportion of passengers with varying dietary challenges, and are providing lower carb and whole grain options, and portion control.
I’m sure you'll be fine, have a lovely time!