Final answer:
Sea ice thickness is primarily limited by its insulating nature, preventing further freezing below and supporting aquatic life.
Step-by-step explanation:
The thickness of sea ice is limited primarily by the insulating nature of the ice itself. Because ice is less dense than liquid water, it floats, forming a protective surface layer that insulates the rest of the water. This insulation allows fish and other organisms to survive in the lower levels of a frozen lake or sea. If ice did not have this property, and instead was denser, it would sink, causing bodies of water to freeze from the bottom up, which would be detrimental to aquatic life. The ice acts as an insulator and slows down the freezing of water beneath it, controlling the overall growth in thickness.