My 12 year old daughter was telling me earlier that you shouldn't drink too much water as your kidneys will not be able to process it fast enough, then I noticed this thread next time I logged on!
I have noticed in some doctor's surgery toilets recently there were several posters saying to tell how dehydrated (or not) you are just look at your wee. If it's transparent then you're drinking more than you need, if it's dark brown you are very dehydrated. Then they gave you all the shades in between. I'm sure there is probably one online if you google it.
Also surely the amount anyone needs to drink is dependent on their weight, activity levels, the temperature that day, what miosis is in the food you eat, etc? That's why the old 8 glasses of water a day thing is particularly inaccurate (in my opinion, feel few to correct me if I'm wrong, I often am).