Most fancy hotels--and I would assume expensive cruise liners as well--will provide, if not a hot water pot in the room, then hot water delivered by a steward, so you can look at foods that can be prepared using that, namely dessicated soups and noodles (if you can get either gluten free).
Also airlines will try and accomodate passengers who need special meals, so I don't know why it would be different for a cruise ship. You need to find the number for their information/liason/concierge and tell them that you have a wheat allergy and can you make arrangements for a special meals? They may have to serve them to you in your room as opposed to the main dining hall/buffet, but that's better than getting sick.
Don't forget too that ships always have a medical facility on board and go to them if you need them. They are used to people having stomach problems due to motion sickness at the very least. Of course after all these outbreaks of stomach flu and whatnot on a lot of them, I daresay they are prepared for worse than that now.
Of course, if they can't accomodate your special diet, it may give you an excuse to slip out of an expensive trip. Vacations are always nice, but not if you have to go into debt for them. Being bankrupt is much less fun. :-(