Great thread, sugahorse!
It's important that people realize that being skinny isn't always the greatest thing. Really important!
I have always been thin myself, and have run into troubles a few times. I am 5'5" and weigh 110 pounds. My doctor says that I am very slim already and cannot lose any more weight. The result has been a nurse checking on me weekly, to be sure I am okay (emotionally & physically). I was put on Boost drinks for about a month when my weight went down to 100. I also had to push myself to eat 3 meals per day.
I have always been a water drinker, which is a wonderful thing (at least I've got that one on my "good list"!

). Having a history in both healthcare and maternity, I do know that 200 mL of water per day is very low. Caffeine is known to deplete the body of fluid as well ~ which is a double whammy for LOTS of people! I know that it sounds like a lot, but I'd highly recommend that you push yourself to drink more water daily. 800 mL - 1000 mL of water per day is what's recommended that adults drink. Drinking 150 mL - 200 mL of milk daily is also helpful with protecting bones from fractures and breaks as we (women) age. Boost also has calcium and lots of other vitamins, which I drank twice per day. It is expensive, unfortunately, but it is very helpful at re-building the body healthily.
Another tip: eat snacks every couple of hours. Doesn't have to be big, or loaded with calories. ie; celery sticks with peanut butter and a Boost. Or an apple, cheese with crackers and a Boost.
You really don't want to be fainting due to low calories and fluid intake. It's not fun (or easy) being IV fed in that state. the nurses have a heck of a time finding arteries when we're that low! I speak with experience on this one, trust me!
Best wishes to you!