It is ubiquitous. It is essential. Without it, life as we know it would be impossible.
Water is a polar covalent molecule, which means that its molecules have a covalent bond that has a charge (polar). The hydrogen (H) molecules have a positive charge and the oxygen (O) molecule has a negative charge, while the whole water molecule (H2O) ends up with a neutral charge. This polarity makes water molecules attract other water molecules via hydrogen bonding.
The hydrogen bonds are weak, so as a result they are broken, and then made all the time. The bonds are broken through kinetic energy (motion). One way to increase kinetic energy is to increase heat. Hotter molecules move faster than colder ones, and as a result are further apart and less dense than their cold counterparts.
As water is heated, the increase of kinetic energy causes the hydrogen bonds to break and the water molecules will change their state from liquid to gas and enter the atmosphere as water vapor.
When water is cooled below 0°C, it changes states by freezing into a solid. Frozen water forms a crystalline structure. Snowflakes, famous for their beauty can only form because frozen water creates crystals. Ice and snow are less dense than water because frozen water expands due to how the hydrogen bonds are affected by the reduced temperature.
Since ice is less dense than water, it floats. This is a life saver for plants and animals because the layer of ice acts as insulation keeping the water beneath warmer, and the plants and animals unfrozen.Water also has a high heat capacity which means that it cools and heats slowly. Functionally, this makes water temperature much more stable than air temperature. This is why, even in the dead of winter, water will often be much warmer than the surrounding air, and even in the dog days of summer, water will be much cooler than the surrounding air.
Water’s high heat capacity is another feature that helps organisms survive the winter. By cooling slowly, the organisms are able to adapt and adjust their metabolism in order to conserve energy when food is less plentiful.
And this is just barely scratching the surface of water.