The oceans cover approximately 70% of the Earth's surface and are apparently unique in the solar system (although keep an eye on some of the giant planets' moons...). The oceans are important for the regulation of tropospheric heat and the storage and production of oxygen and carbon dioxide. [Journal of Geology; v101; 191-213; 1993.] [Geotimes; v39; 12-15; 1994.]
