All living things require water to survive. Being that we are made of around 60% water after the age of 1, its understandable that our day to day water intake really does make a difference in how we look, feel and function.
A good baseline is at least 70-ounces of water for sake of a healthy functioning body; all of our organs and digestive system need water to work efficiently and deliver nutrients throughout our systems, so don't skimp! How active you are and what type of climate you're in will help determine what's best for you, because water also helps the skin regulate the temperature of the body through perspiration (sweat isn't a bad thing). So, if you're being active for 30 minutes or longer, or are in a warmer environment, add in an extra 20-40 ounces of water for that day to help keep your body hydrated.
I make a habit to get at least 20 ounces of water in, first thing in the morning, but my guzzling doesn't stop there. During days I'm more active, in the sun, or eating heavier foods, 120 ounces is my target, which equates to about a gallon. For days I'm not training, or am more sedentary, I'll reduce my water intake 20-30 ounces unless I just happen to be thirsty enough to drink more than that 90 ounce range.
So drink up! Your brain and entire body will thank you.