Most liquids contract as they freeze, making their solid form denser than their liquid form. Unlike virtually every other liquid, however, water expands when it freezes, making frozen water less dense than liquid water. That’s why ice floats. This special property of water helps preserve it in its liquid state. If water acted like most other liquids, oceans and lakes would freeze from the bottom up until there was no liquid left. Most of the Earth would be permanently encased in ice, making life as we know it impossible. The unique way in which water freezes has a special relevance to the continuing existence of large organisms such as human beings.
[T]he freezing of water from the top down rather than the bottom up, which conserves large bodies of fresh water on the earth, is… relevant to large organisms. Bacterial cells can withstand quite well periodically freezing. And for unicellular organisms living in the hot sub surface rocks it’s pretty well irrelevant. In other words the top down freezing and the consequent preservation of liquid water is of much more utility for a large organism, but of far less relevance for microbial life.“Interview with Michael Denton” (SuccessfulStudent.org)
For more information about the unique properties of water, and how they make our lives possible, view The Privileged Species or read Chapter 2, “The Vital Fluid” in Michael Denton’s book Nature’s Destiny.