Imagine a lake filled with fish, plants, and other creatures. In the winter, the top of the lake freezes and turns into ice. What if that ice sank to the bottom, then more ice formed on the top, and this cycle repeated?  Eventually ice would completely fill the lake, killing all the creatures living inside of it.

Most liquids become heavier and sink when they are frozen, but thankfully that is not the case with water. Water expands as it freezes, resulting in ice that is less dense than the water around it.  Because of this anomaly, ice that forms in lakes, seas, and oceans stays on the surface, preserving the wildlife below. 

Our creator gave water this unique quality in order that life would be possible on Earth!

(Source: Inspired Evidence – Donald DeYoung, Weather and the Bible, 1992 pp.60)