In winter season, ice formed on the surface of an ocean or a lake provides thermal insulation. Which ensures the survival of the aquatic life. The reasons for maintaining solid ice at the top and liquid water below ice is/are

Pure water crystallizes at 0°C. In ocean and lakes the water contain dissolved salts. Since pure water crystallizes at the top, the concentration of salts in water increases which decrease the freezing point further. Also the pressure at a depth of 1500 m makes the equilibrium shift from ice$\underset{ }{\rightleftharpoons }$water. The density of ice is less than water.