In addition to its unique chemical properties, water also has some unusual physical properties. Water is the only substance that is commonly found in all three states (solid, liquid, and gas) on our planet. Liquid water is more dense than the liquid form of most other materials. To discover why this is important, think about what happens during an oil spill: the oil floats on top of the water (because the oil is less dense). Therefore, the oil is able to be easily washed up on the shore or travel long distances, damaging sensitive aquatic and shoreline ecosystems.

Did you know that water is the only substance in which the solid form (ice) is less dense than the liquid form? This is important because it allows water to expand when it freezes. In an aquatic ecosystem, ice forming on the surface of lakes and streams insulates the water underneath, enabling organisms to survive cold winters. If the opposite were true - if ice was more dense than water - the ice would sink and lakes would freeze solid in the winter.

Water also has a high specific heat. Specific heat refers to the amount of heat required to raise the temperature of 1 gram of a substance by 1 ºC. What does this mean? It means it takes much more energy and a lot longer to heat water; conversely, it takes a long time for water to cool because it can retain its heat. Because of this property, water bodies can have significant influences over climate and water in the atmosphere helps regulate Earth's temperatures.

Source: http://ga.water.usgs.gov/edu/waterproperties.html