Water is everywhere. Earth is covered by more than 70% water.

Water requires more heat than expected to convert liquid water into vapor. Water is heated and cooled more slowly than expected so in effect it buffers sharp temperature changes. This ability aids life because it assists in keeping the body temperature constant. Homeostasis is the maintenance of a constant internal environment so water helps the homeostasis of body temperature. A normal human body temperature is 37 degrees centigrade or 98.6 degrees Fahrenheit and so the water in the body will be at that temperature.  Therefore if the living environment changes from one extreme to another, hot or cold, then the water will take longer to adapt from the body temperature to the temperature of the environment. This provides a better resistance to fluctuations in environments and increases length of survival.  The human body uses sweat to cool itself, and holds water in to help keep it warmer.

Water is the most important substance in our evolution and our daily lives.  Without water, life would not exist, at least not as we know it.  Every action in a living human cell releases some heat.  If this heat were not dissipated by water, we would literally cook ourselves.  Water is a very useful biological substance; it is vital to both the tissues of the body and of the diet.  About 60% of the water we take into our body come from drinks, 30% comes from the food we eat, and the last 10% is metabolic water (the product of respiration).  Of course there are many things humans need for life, but in order, water is second only to air for humans to sustain life. The human body is about two-thirds, or 72% water.  It is present throughout our entire body. It breaks down for our major organs something like this: Our brains are made up of about 75% water, our heart also about 75%, lungs 85%, liver 86%, kidneys 83%, muscles 75% and our blood consist of about 83% water.  It is necessary for maintenance of all exchange processes. It helps transport nutrients and waste products in and out of cells. It is necessary for all digestive, absorption, circulatory, and excretory functions, as well as for the utilization of the water-soluble vitamins. It is also needed for the maintenance of proper body temperature.

If you add up all the reasons water is essential to human life on earth, which is really much more than I have listed in this paper, you can see the realization of why water needs to be preserved or we will all die.  For all the knowledge we have gained in scientific study about water, there is yet much unknown about its importance.  Water is the most studied substance on earth, and still very poorly understood.  At first sight H2O seems to be an extremely simple molecule of hydrogen and oxygen, but that could not be further from the truth.  Water is much more complex than is chemical formula would lead you to believe.