I worked on RCCL and the ship I was on had 2 messes, one for crew and one for officers and staff but everyone was allowed in both. The food was pretty limited though and very repetitive. If you are quite fussy you might struggle and after a few months you definitely look forward to eating in the ports you visit!
Every night in the officer mess there was salad, steak, chicken, dhal, mashed potato, rice, fish and a couple of other options that changed every day, the crew mess had the same plus food from the Philippines etc. It certainly wasn't the standard of the food passengers got in the dining room
Depending on your rank you can also visit some of the restaurants that are open for passengers and that can add some variety.