You need water when you feel thirsty and not otherwise. Excess water bloats you and is hard on the kidneys. Water does help to cleanse the kidneys, but too much puts a burden on them. When you carry extra water you carry extra weight (which slows you down and increases your work rate), you dilute the minerals in your blood which changes the chemical gradient across cell membranes (intefering with metabolism and thermal control), and your blood volume can rise to a level that stresses your heart. Exess blood volume builds up in the circulatory system and leaks through the vessels into the interstices of the body (the same thing that happens with congestive heart failure).
I could go on, but a more general point is lurking here. A lot of fitness advice is just repeated rather than thought through and it is probably true that most manufacturers go beyond the pale in marketing their products.
