This property of water makes it a very unusual substance. Almost all compounds are denser as solids than liquids. Water molecules, however, as they lose kinetic energy through cooling form ice crystals in which the molecules link up to contain more empty space than what’s typically seen among molecules of liquid water.
The only other substance I know of with this solid-floats-on-liquid property is the metal bismuth.
That water floats has important implications for life. Bodies of water form an ice crust that helps keep the remainder of the volume liquid. Any other kind of liquid would crystallize from the bottom up, eventually freezing totally solid every time air temperature fell. Living organisms must have liquid water to survive.