Upvote:8
You should not expect to choose from a menu or have anything like a restaurant experience, you're simply sharing meals with the crew. In general, everybody's priority is keeping the ship on the move. The officers will be informed in advance that they have a passenger and perhaps your basic info (name, age, citizenship) but other than that, taking care of you is not their job. For example, when and where you board the ship is organized with the local agent from the shipping line, nobody on board is worrying about that.
So it would not seem reasonable to have a separate meal cooked for you. And, in my experience, the food isn't generally vegetarian, far from it. There is also no system in place to communicate special requirements in advance like there might be for air transport.
That said, on my last trip, the cook was very nice and always asked if I liked what he had planned. Since I always said yes, I don't know how much he would have been able to accommodate special requirements but it was always possible to have only salad or (non-vegetarian) soup if you didn't feel like a full meal. Fruit, snacks, or ice cream were also always available (although here as well the snacks included mostly cold meats).