On two occasions last week (on Ocean Princess) I dined with two individuals who had special diet needs. Their diets were special in deed, and not just a preference of taste or weight watching. One of the ladies advised me she had alerted the head waiter when she first boarded, and each night they would take her order for all her meals (B,L,D) for the following day. She had very stringent needs and at the time I met her all her needs had been met thus far. The other lady was ill, and had been instructed by the ship's doctor to eat specific foods with specific preparation. When I sat with her she explained the doctor's order to the waiter, and he arrived at her place setting with poached white fish and steamed vegetables--just as the doctor had ordered.
Personally, I would take all meals in the dining room so there is control over the food and/or preparation needs. Fortunately, I am able to eat what I want so I was most impressed to see that my shipmates were graciously and carefully served.
Have a fabulous time!