Speaking as a self taught dietician out of necessity because of all the food allergies, intolerances, other health issues etc. of the people I regularly feed, it really depends on your height, weight, and physical activity level.Just FYI, eight 8 oz glasses of water is a half a gallon. Does anyone drink this much water every day? I've been trying to drink more water but 4 or 5 glasses is about all I can drink per day before I feel bloated. I wonder if there is any scientific basis for this "recommendation" of 8 glasses per day? I can't find any.
On average 64 ounces fluid intake (8 8-oz containers) is the bench mark but it doesn't have to be in the form of water. The liquid content of veggies, fruit, coffee etc. can also be factored in. A large serving of watermelon can easily contain a cup of fluid. (Watermelon is 80+% water.) People who are more active will need more fluid--generally will WANT more fluid--than those who are sedentary. Four or five 8 ounces of water in addition to everything else is probably okay.
But it isn't okay to forego water entirely. Those who get most or all of their liquid intake from tea, coffee, soda, juices, milk, beer and such won't die of dehydration but will not have optimum health either.