Ice
Because ice floats, even large bodies of water which freeze, like some oceans, only form ice on the surface. Most lakes never freeze to the bottom. Even the coldest oceans, like the Arctic Ocean, only freeze on the top, leaving liquid ocean circulating below. Because of this the Earth's oceans are able to redistribute heat. The climate of the Earth has less extremes of heat and cold than it would otherwise. A solid ocean would not circulate heat. But because ice floats on the surface the water beneath can continue to circulate. Ice on the surface stays exposed and readily melts when the temperature rises.