Yes, you should be drinking 6 to 12 glasses daily--read the book, or the FAQs above for all the details you should have read up on before starting! :)
And you ARE doing a wonderful kidney flush... the Master-Cleanse is a cleanser of all things internal--your pancreas, kidneys, liver and gallbladder, as well as your intestines etc.
You might be thinking of the liver flush--go to the forum here at curezone and read up! My naturopath has me eat a good rebuilding diet for a couple of weeks to recover fully from fasting before flushing my liver.