When water changes to ice, it becomes lighter.
It is the only substance on earth that is lighter as a solid than as a liquid. This means that when the water in a lake freezes, the ice floats to the top, and fish swim beneath it.
It is very, very rare for a lake to freeze into a solid block of ice. But this is exactly what would happen if ice were heavier than water. Ice would form in a lake from the bottom up.
During a cold winter, ice would build up and fill the entire lake, and the fish would be trapped and frozen in it.
Fish couldn’t survive from one year to the next. Life in the water as we know it could not exist.