I wrote this in this thread:
http://boards.elitefitness.com/forum/showthread.php?threadid=251523&highlight=water
There is a balance to be stuck here, hydration is good, but anything to excess causes problems for your body. It is possible to consume too much water, which if combined with excess sweating (like if you are taking ECA for example?) can lead to Hyponatremia or "water intoxication”.
As you consume large amounts of water over the course of a day, blood plasma (the liquid part of blood) increases thereby diluting the salt content of the blood. At the same time, your body also loses salt by sweating. Consequently, the amount of electrolytes available to your body tissues decreases over time to a point where that loss interferes with brain, heart, and muscle function! You have to replace these electrolytes! They're essential to the normal electro-chemical operation of your nervous system.
Though the condition is opposite to dehydration, the symptoms generally mirror those of dehydration (apathy, confusion, nausea, and fatigue), although some individuals show no symptoms at all.
That said, you do have to consume a huge amount of water and sweat a lot to get into this state, it is more associated with long distance runners who do not eat and therefore do not replace the lost electrolyte’s, than strength athletes. Just to be sensible! I’m no expert, but I would say that 2 or 3 gals is fine, however I cant see why the body would need 4 gallons or more in one day.