Water and Density

As the temperature of fresh water decreases, the molecules move closer together and it gets more dense. Water is most dense at 39˚F. But, once water freezes at 32˚F it actually becomes less dense due to the crystal structure of ice. This is the reason ice floats, and is great news for life on Earth. If frozen water did not float, bodies of water would freeze from the bottom to the top and no life would exist in them.

Source: Under Artctic Ice | Smithsonian