For most substances, the solid state of matter is denser than the liquid state of the same substance. As atoms and molecules slow their movement at low temperature, they get closer together. Generally, this increases the density of a substance.
Water is one exception to this rule. In liquid water, some molecules are bound together by hydrogen bonds but some molecules move freely. As liquid water freezes and hydrogen bonds form between water molecules, those hydrogen bonds form a geometrical pattern called a lattice. The fixed geometry of the lattice prevents water molecules from packing close together and, therefore, from becoming denser as it freezes. Instead, water expands upon freezing.
This is why ice floats in cold drinks and why liquid beverages cooled in the freezer may explode. Similarly, in the polar regions of the globe (high latitudes), ice forms over parts of the polar seas during cold seasons. The diverse marine life that lives in those oceans can survive cold seasons because ice floats on top and insulates the deeper water against colder atmospheric air.