Most liquids become heavier when they solidify.
Water, however, becomes lighter when it freezes. Its point of highest density is about 39° Fahrenheit (4° Celsius), which is 7° Fahrenheit (-14° Celsius) above freezing. Therefore, ice weighs less than water and floats.
If ice didn’t float, it would sink to the bottom of the lake or pond. Even in warm months, the Sun would not be able to melt it through the surface water, so more and more ice would form.
Over time, a pond or lake would become a solid mass of ice with just a few inches of water on top. Nothing could live in it.