Huh.

The American Enterprise: Are We A Privileged Planet?:

One thing we learn right away in elementary physics is that gas is the least dense state of matter, liquids are in the middle, and solids are the densest. That's because the molecules are loosely associated in gases, adhere together somewhat in liquids, and are tightly bound together in solids.

There is one glaring exception, however - ice. Unlike any other element or compound, H2O is lighter as a solid - ice - it is as a liquid - water. No other substance has this property. Is this a big deal? It certainly is. It just so happens that it made the evolution of life possible.

If ice were heavier than water, it would sink to the bottom in a lake or shallow sea. Then, more water would freeze on the surface and sink again and soon the whole body of water would be frozen solid from top to bottom. Anything living in that lake or shallow sea would die. Since most life originated in water, living forms never could have survived.

Instead, ice floats. Why? There doesn't seem to be any real explanation. I've always thought it was evidence that God was willing to admit His mistakes and bend the rules when it counted. When He was finished designing the fundaments of the universe - gases, liquids, and solids - He said, 'Oh, darn, I forgot. This isn't going to work.' So, He made that one exception. All solids shall be denser than their liquids except water. That way life could evolve.