Final answer:
The key property that enables ice to float and protect aquatic life underneath is its lower density compared to liquid water, due to water's unique molecular structure when it freezes.
Step-by-step explanation:
The key property of water that protects aquatic life underneath ice is its unusual density properties when it transitions from liquid to solid. Specifically, ice is less dense than liquid water, which is why it floats on the surface of ponds, lakes, and oceans. This phenomenon occurs because of the unique molecular structure of water that forms a hexagonal lattice when water molecules freeze, taking up more space and thereby decreasing in density. As a result, ice acts as an insulator for the water beneath, maintaining a habitable environment for fish and other organisms throughout winter. In contrast, if ice sank as it froze due to being denser than liquid water, bodies of water would freeze from the bottom up, endangering aquatic ecosystems.