Water is a tool, a major component of your body and essential for life. You can exist without food for a few weeks, but go without water for a few days and you're done. It is part of everything; blood, digestive juices, perspiration, lymph, body fluids, muscle, brains, everything - even bones. It also enables food nutrients to enter cells, flushes out waste, regulates body temperature, gives a fetus something to bounce around in, and is crucial for your thousands of biochemical processes going on all the time.
Given the necessary raw materials, your body will find the correct balance for what you need to live. But while many things can provide the liquid you need, there is no substitute for pure clean water. For example: a can of soda pop is mainly water but some of its ingredients cause your body to become more acidic. You then have to draw material from your body to regain the correct acid/alkaline level (this could be calcium from your bones) and a large amount of water is required to re-balance. To hydrate effectively, keep drinking water throughout the day whether you are thirsty or not (it will also help maintain a "full" feeling and perk you up if you're feeling zapped). If you wait till you're feeling thirsty dehydration is already starting.
