Water has many unexpected and remarkable properties. Its molecular weight would suggest that its boiling and freezing points should be lower than they are and based on molecular weight water should freeze at -100oC and boil at 80oC. However its boiling point and freezing point are 100oC and 0oC. In addition water occurs on earth in its three natural states solid liquid and gas and there are no other substances on earth that exist in all three states like water does. Water at 4oC beings to expand instead of contract like other substances, and at 0oC it becomes ice a solid occupying a volume approximately one eleventh greater than its volume at 4oC. As a result when in water floats upon itself.
If the reverse were true and ice did not float, water would freeze from the bottom up and thus as a solid mass destroy all life within the water as it freezes. This would have dire consequences on the environment as frozen masses of water would remain frozen from season to season dramatically changing the climate and weather. Evaporation would be less and thus precipitation would also be less. Some places would be stark and arid due to a lack of water and thus temperatures would fluctuate greatly from each day to night. Days would be extremely hot and night would be extremely frigid without the temperature moderating effects of water.