All around us is a little molecule that is found in the air, makes up all the rivers, lakes, and streams, and is more important to life than any other. This little guy is a simple combination of two hydrogen atoms and one oxygen atom, but is so important to our existence that when scientists are looking for worlds that might have life, they look for water first. No water, no life. Water, maybe life.
The fact is that water makes up about 70% of our bodies. Cells are mostly bags of watery stuff floating around. Some cells have more or less, but some, like blood cells, are made of as much as 90% water. Most of this water is a mixture of substances within cells are a common molecule existing within a larger one, but there is nowhere that water alone is stored in the body with the exception of the bladder. And of course it is only stored here until excretion and is still a combination of water and other waste products, although it is more than 95% water.
Most of the reason that water is so important is that it is unique. No other molecule has quite the properties that water does. Since water is a combination of two hydrogens and two oxygens, it is shaped like a triangle, but oxygen atoms attract electrons better than hydrogen does. Chemists call this electronegativity. So the electrons in a water molecule hang out nearer the oxygen, causing the whole atom to have a slight negative charge toward the oxygen, and subsequently a tiny positive charge near the hydrogens.
When a bunch of water molecules get together, they line up with oxygens toward hydrogens and kind of stick together a little bit. This creates surface tension and is the reason that if you fill a glass of water to the top and slowly add a little more, you can see a bubble of water slightly above the top of the glass. The water is holding itself together through surface tension. Try this with alcohol, hydrogen peroxide, or just about any other liquid that doesn't have water in it, and it will not work.
In the body this becomes important because if water did not have this property, it would boil at room temperature. The surface tension resists boiling and so it stays a liquid up to far greater temperatures than it would without this property.
Water also resists heating and cooling. It takes far more energy to change the temperature of water than many other liquids, which is important for us humans, because our bodies only function properly within a very small range of temperatures. If our body temperature fluctuated a lot, we'd be in a world of hurt.