According to the Water Management Society, they say that:

It is a myth that eight glasses of water per day is essential to good health. The source of this myth was the 1945 recommendation by the US National Research Council (NRC) that adults should consume 1 ml of water for each calorie of food. This works out as 2.5 litres per day for men and 2 litres for women. At the time the NRC pointed out that this includes the water in food so only about 1 litre of water needs to be drunk. This does not need to be pure water as the liquids that people drink such as tea and coffee provide all the water needed.

Another part of the myth is that caffeinated drinks do not apply as they are diuretic. This is not correct as a comparison of healthy adults in 2000 found no difference in hydration whether the water was from caffeinated drinks or not. Even one or two low alcohol drinks will hydrate you. Drinking too much water can be harmful. Unsurprisingly, bottled water companies perpetuate these myths but they are also propagated by many doctors and health organisations.