A cubic foot of water weighs 62.4 pounds and a cubic foot of ice weighs 57.2 pounds. This makes ice lighter than water.
Water expands as it freezes. It increases by almost 10 percent in volume. Because of this, ice is lighter than water and takes up more space, which is why you can fill up an ice cube tray with water and the ice cubes will take up more space in each compartment than the water did.
When lakes and ponds freeze over, it is entirely possible for aquatic life underneath to survive. The ice forms a blanket for the water and keeps it at a higher temperature than the outer layer.