The body uses a lot of water during fasting, and the amount you need may vary.
Here's a very strange trick I learned and have used for dry nasty mouth from time to time while fasting:
There is a whole forum on this ayurvedic therapy here at curezone. Weird? You bet! But when I tried it I thought immediately of dry mouth while fasting, and it has helped me.
Put a tablespoon of good quality oil (I like Hain cold pressed walnut) in your mouth. Obviously as you are fasting you religiously avoid swallowing any. Start to swish it through your mouth. Don't get excited--this takes time! Swish, roll, chomp the oil! Do it some more! Strive for ten minutes (I like to work on the computer). At the end, spit it ALLLL out into the potty (supposedly it can be toxic, so flush it), and proceed to rinse your mouth with water til all oil is out. This won't be tough because after ten minutes it has emulsified very well with your saliva.
I have found this very soothing and lubricating to a dry mouth while fasting. You might give it a try!