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The property of water that is essential for maintaining life in aquatic environments during winter is its less dense solid state. When water freezes, it expands and becomes less dense than liquid water. This unique characteristic allows ice to float on the surface of bodies of water, creating an insulating layer. As a result, the water beneath the ice remains liquid and can maintain a stable temperature, providing a suitable habitat for aquatic organisms during cold weather. This phenomenon is crucial for the survival of fish and other aquatic life, as it prevents the entire body of water from freezing solid.

Other options do not play the same critical role in the context of winter survival for aquatic life. For instance, water's high boiling point is related more to its ability to remain a liquid at higher temperatures rather than its behavior during freezing. The high specific heat of water helps to regulate temperature changes in the environment but does not directly influence the formation of ice and its consequences in winter. Strong ionic bonding, while important for the structure of water molecules, is not a relevant property for maintaining life in aquatic ecosystems during colder months.