I am not an expert, but I have heard different amounts over the years.
One included an equation based on your weight to give you a base amount of water, then adding 1 glass for every carbonated or caffeine beverage you consumed, and an additional glass for every so many minutes of exercise to replace fluids.
At one time I noticed that some of the more intense people at the gym weighed themselves before the work out and then again after, replacing fluids to bring their weight back to the starting point. I doubted that until the day I saw a puddle around one of the exercise machines. One guy had sweat so much that you could see it just dripping off of him.
I also heard that women need different amounts of fluids during hormonal fluctuations during the month. I understand that with water retention. If I take in 8 a day, I am less likely to retain fluids in my hands and feet.
I have also heard that the 8 a day can include liquids used in cooking. Herbal teas are good as well.
I have also learned that too much water is a problem. My sister-in-law was rationed. Her doctor told her to keep in under 16 a day. As for water poisoning, I looked that up and you have to take in a lot of water in a short period of time. It would be pretty intense.
So that is everything I have heard about water. I try to keep it to my 8 a day.
July 3, 2014 - 1st day home after MI
Nov 14, 2014 Lowered my BMI by 4.1
1st Goal - remove an additional 2.7 from my BMI
2nd Goal - remove 5 more from my BMI
3rd Goal - remove 5 more from my BMI
Semi Final Goal - remove 5 more from my BMI