Water is a polar molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge (δ-) near the oxygen atom due to the unshared pairs of electrons and partial positive charge (δ+) near the hydrogen atoms.
The shape of the water molecule is triangular rather than linear; the angle between the nuclei of the atoms is approximately 105˚.
In theory, water should be a gas, but, at room temperature, it is a liquid. The hydrogen bonds in water hold the molecules together, causing it to be liquid.
Water has a high heat capacity; that is, a great deal of heat is required to raise the temperature of water. This, again, is due to the hydrogen bonds. Most of the energy is used to break these bonds, restricting the mobility of the molecules. As a result, water takes a long time to heat and cool. This specific heat capacity is the highest of any known substance. This is significant because it means aquatic environments are slow to change temperature. It also means bulky organisms have a stable temperature in the face of fluctuating external temperatures.
The heat energy needed to melt ice, known as the latent heat (enthalpy) of fusion, is unusually high, as is the latent heat of vaporisation (energy needed to vaporise water). This, again, is due to the hydrogen bonds and the polarity of the molecules. Relatively large amounts of heat must be extracted from water before it freezes. This is advantageous to living things because the contents of cells and aquatic environments are then slow to freeze in cold weather. Looking at the high latent heat of vaporisation, much heat is lost by the evaporation of a small quantity of water, and so evaporation of water in sweat or transpiration causes marked cooling.
When water is ice, surprisingly, it is less dense than liquid water. This is useful in the sea, lakes and ponds because it means any ice that forms in cold weather will float. The ice insulates the water below and so it less likely for the whole area to freeze over. As water is colourless, it also enables plants to photosynthesise well in deep water.
In humans, the low viscosity, or runniness of water is essential, as it lets water flow easily through capillaries.
Certain small animals rely on water to land on and to move across. This is because at the surface of water, the molecules are positioned so that the hydrogen bonds point inwards, giving water a very high surface tension. This enables the lighter creatures to move on top of the water surface.
All these properties and uses of water show that it an unusual and incredibly important substance in biology. Life as we know it depends on these properties.