Just a warning. The Nautical Spa menu selections are supposed to be healthy, but they are anything but low fat. They actually call Alfredo sauce healthy. The lobster tail is probably one of the healthiest options. Use common sense when selecting. You'll recognize healthy food better than Carnival will.
There is a salad bar, but on the Victory it was very small and had no fat-free dressing. I understand that they usually do have it though. The veggie burgers are great and they have frozen yogurt. Try stuff without a lot of sauce. Just remember, all the food is great. By choosing something healthy, you won't miss out on anything.
There are cold cereals and skim milk on the breakfast buffet, but for some reason, I couldn't get them to provide skim milk when I ordered room service. I always got whole milk.