There are a lot of properties unique to water that actually makes it essential to life (on this planet at least). I don't know of another liquid that has as many of these properties that would make it suitable. A few of these properties are:
- High specific heat. Specific heat is the amount of energy required to change the temperature of a substance. Because water has a high specific heat, it can absorb large amounts of heat energy before it begins to get hot. It also means that water releases heat energy slowly when situations cause it to cool. Water's high specific heat allows for the moderation of the Earth's climate and helps organisms regulate their body temperature more effectively.
- Water conducts heat more easily than any liquid except mercury. This fact causes large bodies of liquid water like lakes and oceans to have essentially a uniform vertical temperature profile.
- Water molecules exist in liquid form over an important range of temperature from 0 - 100° Celsius. This range allows water molecules to exist as a liquid in most places on our planet. This is important due to water's important use as;
- A universal solvent. It is able to dissolve a large number of different chemical compounds. This feature also enables water to carry solvent nutrients in runoff, infiltration, groundwater flow, and living organisms.
- High surface tension. In other words, water is adhesive and elastic, and tends to aggregate in drops rather than spread out over a surface as a thin film. This phenomenon also causes water to stick to the sides of vertical structures despite gravity's downward pull. Water's high surface tension allows for the formation of water droplets and waves, allows plants to move water (and dissolved nutrients) from their roots to their leaves, and the movement of blood through tiny vessels in the bodies of some animals.
- High latent heat of evaporation gives resistance to dehydration and considerable evaporative cooling.
- Unique hydration properties towards biological molecules (particularly lipids, proteins and nucleic acids) that determine their three-dimensional structures, and hence their functions, in solution. This hydration forms gels that can reversibly undergo the gel-sol phase transitions that underlie many cellular mechanisms.
- Water ionizes and allows easy proton exchange between molecules, so contributing to the richness of the ionic interactions in biology.
- The density maximum at 4°C and low ice density means that all of a body of water (not just its surface) is close to 0°C before any freezing can occur. Also the freezing of rivers, lakes and oceans is from the top down, so insulating the water from further freezing and allowing rapid thawing, and density driven thermal convection causing seasonal mixing in deeper temperate waters.