Your body is about 50–80% water. When you're in need of water, you'll probably get a headache, lose concentration more easily and your mood might also take a turn for the worse.
Without water, your balance of essential minerals would be shot and, with a gastrointestinal tract devoid of moisture, good luck digesting and processing your food or eliminating waste. You can also forget about your ability to regulate temperature when you get hot.
The rule of thumb for your daily water intake is 35 millilitres of fluid per kilogram of bodyweight. So, for a 70kg person in everyday circumstances, 2.45 litres would be about right. That same amount could be lost in a single hour by someone exercising in extreme heat. You can lose between 1 and 3 per cent of your body's water quite easily.