There is a wealth of information about dehydration if you google "symptoms of dehydration".....quite numerous!
Dry fasting is abstaining from water & food, but of course the body can live much longer without food than water, and even less without air!!
However, my own view, which has worked for me, is to drink water while fasting according to thirst, just as you would eat food when you are hungry.
I think we can safely say that we can rely on the bodies innate wisdom to know if fluids are needed or not, just as you would rest when tired or the bodies demand for sleep when sleepy.
Try eating a three course dinner after dinner!!
Or drinking twenty pints of water in one go!