Water makes up more than two thirds of human body weight, and without water, we would die in a few days. The human brain is made up of 95% water, blood is 82% and lungs 90%.  Water also plays a crucial role in the bodies ability to regulate temperature. Our bodies can control over-heating through perspiration from sweat glands in the skin and from evaporation which produces a cooling effect. Blood is also routed into areas close to the surface of the skin where it can be cooled and then carried back to the interior of the body. A mere 2% drop in our body's water supply can trigger signs of dehydration: fuzzy short-term memory, trouble with basic math, and difficulty focusing on complex task.  During a disaster when stress is high and the ability to focus is crucial even mild dehydration can turn disastrous. Water helps our bodies remove toxins in many different ways. Water flushes toxins and waste from the body through urination and perspiration. Water helps reduce constipation and aids in bowel movements which ensures that wastes are removed quickly and regularly before they can become poisonous in the body. This waste buildup can occur in the body if dehydration becomes a regular occurrence and this can cause headaches, toxicity and illness. Getting sick during a disaster when medical care and access to antibiotics is going to be limited or non existent can quickly turn fatal. 

I think it is quite clear how clean and drinkable water is a critical  resource during a disaster and things can quickly go from bad to worse without it. Clean water is the lifeblood of human existence quite literally.