While it is good to drink water, you are not really 'flushing' your kidneys. Much of the time, you are overloading them. Drinking vast amounts of water at a time can put a strain on them. Patients with kidney problems are told not to drink more than a cup of liquid at any one time. Experts tell us not to wait until we are thirsty to drink.
Dehydration is bad news.
You feel achy, tired, sluggish. Many people don't drink any water. They think that if they aren't hot and sweating they aren't losing water. You lose water just by breathing. Breathe on a mirror and see the water vapor you make.