As mentioned above, your body is about 60-percent water. A person at rest loses about 40 ounces of water per day.
Water leaves your body in the urine, in your breath when you exhale, by evaporation through your skin, etc. Obviously, if you are working and sweating hard then you can lose much more water.
Because we are losing water all the time, we must replace it. We need to take in at least 40 ounces a day in the form of moist foods and liquids. In hot weather and when exercising, your body may need twice that amount. Many foods contain a surprising amount of water, especially fruits. Pure water and drinks provide the rest.