I know getting in all the water is hard, but at this early stage, it is THE single most important thing to do. Quite frankly, your body will not waste away from malnourishment if you don't get enough protein or vitamins for a few weeks, but if you don't get enough water early out, you could very well end up requiring IV fluids due to dehydration (I did, and I have spoken to many others who have as well). Dehydration SUCKS and makes you not want to eat or drink ANYTHING (seems counterintuitive, but it's true).
So, focus on water as your #1 priority right now. (That does NOT mean to ignore everything else, but if you are truly struggling to get everything in, water is most important right now.)
As for vitamins, as has already been mentioned by others, just keep your calcium and iron apart, preferably by at least 2 hours, and don't take your iron with anything containing calcium (such as a protein shake, dairy, etc.), but DO take iron with Vitamin C. Other vitamins, for the most part, will play nice together, so you can take things like a multi and calcium at the same time.
Highest known weight: 324
Weight on morning of DS surgery: 308.5
Got down to about 185 before pregnancy;
Benjamin David born March 24, 2012, 7 pounds 11 ounces