I believe someone once said drink 8-10 glasses of fluid a day, not necessarily water. This has been changed over time to 8-10 glasses of water, and this has been bandied about ever since, in every mag, book, tv programme, etc etc.
I used to believe that too, and i tried to drink that much but found it so difficult,it did not come naturally or easily to me, as i'm not a water person. I prefer tea, herbal, if being good, as i'm a hot drinks sort of person. My son will drink lots of water, and rarely touches soda type drinks and never has hot drinks, so i do think it depends on your type. Homeopathy puts people in types, hot or cold drinks people, etc etc.