Unlike most liquids, water freezes from the top down. If it did not act in this unusual way, all life on earth would eventually die:
“Streams, rivers, lakes, and oceans freeze from the top down, because water reaches its maximum density at 39°F—seven degrees above its normal freezing point. As cold air further lowers the water’s temperature, water defies the behavior of most liquids and expands. This less dense water “floats” on top of the denser water. Eventually, it freezes into ice, which is even less dense.
“We are fortunate that water behaves in this unusual way. If water continued to contract as it became colder and froze, as most substances do, ice would sink. Bodies of water would freeze from the bottom up. Surface water would quickly freeze, then sink. During the summer, the overlying liquid water would insulate the ice and delay its melting. Each winter more ice would collect at the bottom. This would first occur at polar latitudes, but over the years would spread toward the equator as surface ice reflected more of the Sun’s rays back into space, cooling the earth. Sea life would eventually cease. Evaporation and rain would diminish, turning the land into a cold, lifeless desert.” (Walt Brown, Ph.D., In the Beginning – Compelling evidence for Creation and the Flood, p. 186, note 124.)