A normal body needs at least 2-3 liter of water per day. If you don't get enough water during one day you'll probably feel the infirmity. Experts says that your capacity will be reduced with 50%.
Normally there's no problem to find fresh water in the mountains. But if you are out in the forest you could be forced to clean your water before drinking it. You can buy cleaning tablets in a drugstore or build your own cleaning system with materials from the nature.
When you boil the water all of the microbes are killed but the chemicals are still left.
Don't try to drink water from the sea (with salt).
We know that a human being can live without food in 2-3 weeks but as you probably have noticed the blood sugar will be reduced very much in half a day. Your body begin to feel a bit chilly and perhaps a small headache is coming up. And worse is that your ability to make the right decisions aren't so good.
Well, it isn't that worse, get yourself a real meal to put your body in balance again.