Water as a medium on this planet also serves a role environmentally. It is very stable as temperatures change because of the hydrogen bonds between the water molecules. If you think of heat as energy that increases the motion of atoms and molecules, you won't be far from a technical definition. When water absorbs heat, the molecules do move faster but the "glue" of the hydrogen bonds slows the process. Heat is transmitted slowly through water, and large bodies of water take a long time to heat up. That energy does not come back out quickly if the temperature around the water drops, either, so masses of water cool more slowly than other liquids (or the air) would.
Masses of water also are stable in that individual molecules that have picked up heat / motion have a hard time getting free into the atmosphere, or evaporating. There are two reasons for this: the first is that heat is passed around fairly evenly (if slowly) and so the number of molecules going fast enough to zip free rises slowly; the second is that hydrogen bonding at the water-atmosphere surface pulls molecules at the surface more tightly together (this produces the surface tension that lets you put slightly more water into a glass than its volume - you've seen the "dome" of water on top? - or that bugs walk across without sinking) and leaves less room for potential evaporating molecules to squeeze through. This also explains why evaporation is a cooling process (useful in sweating, or dogs panting): what is lost during evaporation are the very fastest, very hottest molecules, leaving what's behind cooler.
Connected to the above property is the wide range of temperature in which water is liquid; although life is tricky at extremes near freezing and boiling, it is possible, as long as water remains liquid.
Another fairly unique property of water is how it solidifies: if water cools, its molecules move more slowly, collide more rarely, and tend to pack more closely together. Like most substances, water gets more dense as it cools. However, when too crowded, at about 4º C, the repulsions among the tightly-packed bipolar molecules cause them to slip into an arrangement which, as the temperature drops, actually pushes them further apart into kind of an expanded crystal arrangement. We all know that ice floats; what this means is that water in its solid form is less dense than water in its liquid form. If ice did not float, it would freeze, sink, and expose more surface to freeze, and sink, and frozen bodies of water would be frozen solid from bottom to surface, a very poor environment for living things and a difficult task to thaw. In fact, floating ice acts as an insulator to the water underneath it. The thicker the ice, the harder it is for the water to lose heat and freeze, so very few deep bodies of water, even in the coldest climates, are totally frozen.
Heat and temperature concepts.
More on water and heat.
Evaporation and condensation (but ignoring surface tension). (Video)
Ask a Scientist: Evaporation and cooling.
Evaporation from a drop on a surface. (Video)
Some like it hot.
How water cools and freezes (Video).
Liquid lakes under Antarctic ice sheet.