dstein12,
so sorry you had to deal with unsolicited parenting advice in addition to the worries of a sick child! I also wanted to mention that my I had a very picky eater -- at age 18 months he was seeing nutritionists because he wouldn't drink anything other than water, wouldn't eat anything other than crackers. Finally he started eating cheese and yogurt and slowly improved, though he was still small for his age (after being a very healthy chubby baby). Once we went on a trip and took crackers and spray cheese everywhere for him just to try to fatten him up a bit. Then at age 4 he went on a cruise weighing 27 lbs. and came home weighing 31 lbs! Nearly a 15% gain in 7 days! I think it had a lot to do with the delicious chicken fingers and fries by the pool as an afternoon snack, cheese and crackers before a late dinner, lingering over dinners that had all his favorites and attentive waiters who really wanted him to enjoy the food. And by the time he was age 5, he got over his extreme pickiness and now several years later, he has a much more varied diet and will at least try everything we serve him.
Now he was definitely not as picky as your daughter, nor dealing with severe medical issues. But food is everywhere on the cruise ship and it is all kid-friendly stuff. Hopefully it will tempt your daughter as well. And if not, the cruise staff will get those Uncrustables to you and keep them coming, especially if you notify them in advance with a doctor's note (so they can stock up, haha). My advice is to go now, don't let the food be the reason you don't cruise!