Water has some remarkable properties. It has almost the highest specific heat capacity of any substance, meaning that it can absorb a lot of heat-energy when its environment is hot, and release a lot of heat-energy when its environment is cold -- making it the primary regulator of surface temperature on Earth. It becomes less dense as it solidifies, so that water habitats in cold regions will remain partially liquid as solid ice floats to the top of the water column and forms a protective layer -- allowing aquatic inhabitants to survive frigid winters. When water molecules form they adopt a tetrahedral geometry, one of the consequences of which is formation of a partial electric dipole -- making water an important solvent. In fact, water is called the 'universal solvent", because it dissolves more substances than any other liquid.
The source of many of water's remarkable attributes is due to its molecular geometry -- the partial electric dipole formed by its two hydrogen moieties and one oxygen atom. Such that the oxygen atom is slightly electro-negative at one pole, and the covalently bonded hydrogen atoms are slightly positively charged at the other pole. This causes water to form tetrahedral-oriented weak bonds with adjacent water molecules. It is for this reason that water molecules are strongly attracted to each other, meaning that it takes a good amount of energy to get them to separate, such as in evaporation or the increased distance associated with freezing.
This weak bonding between hydrogen atoms and electro-negative elements is called hydrogen bonding, which gives water its amazing qualities but is also involved in many other molecular interactions -- such as holding DNA strands together to form the double helix, coordinating protein folding to enable nearly every function of the cell, and causing stable membranes to form due to hydrophobic--hydrophilic interactions. It is such an important interaction that if the strength of hydrogen bonding were even slightly different in water then life as we know it would not be possible.