The molecule of water is one of the most important molecules on the planet. First of all, several hypotheses concerning the origin of life put water in the central role: since water is present in all living beings (except a virus, which is not held in the scientific community as being for sure a living being), it is supposed that life must have started in water somehow.
Water also has a very important role in regulating the temperature of Earth, both water vapour in the atmosphere and water currents. We could also mention its importance to generate electricity in usines.
But, most important of all, water is an essencial molecule for life. Pratically all the reactions that take place in the human body use water as a solvent. Water is also an important component of our blood, which carries O2 troughout our whole body. Without water, much of some important reactions in our body would just stop, and we wouldn't be able to carry O2 to our tissues.
A very good example of a reaction that is extremely important to life as a whole, and which demands water, is the photosynthesis:
This reaction is responsible for the production of O2 in the atmosphere, which we, human beings, as well as several other mammals, need in order to properly breath. This reaction also eliminates CO2 of the atmosphere, a gas that can be a problem due to the greenhouse effect. Even though it is not directly happening within the human body, it still as crucial as a reaction that happens in our body: no water, no photosynthesis. No photosynthesis, no oxygen.