Use common sense:
How much water does the human body need? Depends. A 200 pounds man needs more than a 50 pounds child, obviously. If the temperature is high or the person is doing an intensive workout, water intake has to increase proportionally to make up for losses.
Without going any further, you know already that it is impossible to set a fixed amount of water intake that will work for every one in all situations.
From there, you can determine beyond doubt that the "8 glasses" recommendation can not be anything more than a guideline, and not a good one at that.
As many individual factors come into play (including body type, climate, activity, water content of food, etc.), establishing a standard amount of water intake as being good for everyone everywhere is idiotic.
Thirst AND hunger make for a much better rule of thumb. Note that I say "hunger" as well, as empirically, you will often find that a sweet tooth craving can be satisfied with water.
PS: for those who abdicate any right or ability to follow a line of reasoning without having to refer to an authority, here are a list of references:
and of course, the obligatory reference to Wikipedia:
They are probably quite relevant to the mater, yet strictly unnecessary to answer the question. But, hey, if you need somebody else's approval to think, go ahead.
Skepticism reduced to reference hunting... Oh, sweet irony.
Skepticism was funded on the observation that truth is not an absolute and that authorities and belief could not be relied on.
And there we are on "skeptics", running Google searches and copying other people's supposed research, because god forbid we would be using our reasoning powers and already available data to come up with even the most simple conclusions.
If that's right, then perhaps I don't belong here.