I had the same concern. I also was worried about not being able to hydrate well, especially when working out. Here's what I have to say about both topics:
1) on the nutrient issue... In the beginning, it'll be tough, but you'll learn you can survive on very little calories. It's super Important that you start Protein shakes and taking your Vitamins as soon as you are able to. I remember feeling amazed that some days I'd eat 500 Cals or so and still be alive! At first you really do feel run down, but that goes away with time, especially once you are able to start eating real food. I try to eat protein as much as possible, but end up eating my fair amount of carbs too. Once you're further out, you gotta just figure out what works for you. I eat pretty much whatever sounds good, work out, and am still losing. Slowly
2) Water.... I am 10months out. I believe you will never be able to eat and drink at the same time. This makes hydration hard for me. I try to drink water as much as I can, but I end up drinking coffee half the time.... I could be well hydrated, if water was all I drank. So you can do it, and I certainly make an effort when I'm working out or feeling crummy.
So far no real complications with either issue. 10 months out and 88 lbs down! Good luck!