Your body will does not store significant amounts of water (most of the water in our body is in blood plasma). Unlike energy which can be stored as fat or carbohydrates water has no real storage mechanism. This is why you will die of thirst long before you die of hunger.

As such once your body reaches it's optimal hydration point point it will expel excess water (you go to the loo). Your kidneys control this process. If you ingest large quantities of water your kidneys will react to expel the excess. No storage will happen. 

This can also put extra strain on your kidneys and lower your blood salts level. So even if this was practical it'd be ill advised. Drinking far too much water in a short **period of time** (remember it's not stored, your kidneys will eject the extra water as fast as they can) can lead to [Hyponatremia](https://en.wikipedia.org/wiki/Hyponatremia) or (**in extreme cases!**)[water intoxication](https://en.wikipedia.org/wiki/Water_intoxication) and even (eventually!) death! Though you would have to this to an incredibly extreme level to get anywhere near the Water Intoxication level. Hyponatremia can be nasty though and is reasonably easy to trigger:

> 13% of the athletes who finished the 2002 Boston Marathon were in a
> hyponatremic state
...
> 
> Signs and symptoms of hyponatremia include nausea and vomiting,
> headache, short-term memory loss, confusion, lethargy, fatigue, loss
> of appetite, irritability, muscle weakness, spasms or cramps,
> seizures, and decreased consciousness or coma

You lose water naturally though sweating, urine, moisture evaporated from your lungs, etc. (you also produce water when you convert glucose to energy but this is a small amount in the scheme of things).

You need to replace lost water constantly. The key to optimal hydration is little and often, because too much and your body will expel it, but too little and you become dehydrated. There's no way to avoid this mechanism.

Related:

**[How can I tell if I am dehydrated or have low electrolytes (need salt, etc.)?](http://outdoors.stackexchange.com/questions/6319/how-can-i-tell-if-i-am-dehydrated-or-have-low-electrolytes-need-salt-etc?rq=1)**

**[How do I achieve/maintain optimal hydration?](http://outdoors.stackexchange.com/questions/9402/optimal-hydration?rq=1)**