The largest body on the surface of earth is called Ocean and an adjacent smaller body of water is called Sea.
Oceans and seas are the bodies of salt water that cover about 71% of the Earth's surface; they are referred to in total as the world ocean. Several centuries ago the "seven seas" were considered the navigable oceans. They were the Atlantic, Pacific, Indian, and Arctic oceans; the Mediterranean and Caribbean seas; and the Gulf of Mexico. At present, however, oceanographers consider all other oceans and seas as belonging to the Atlantic, Pacific, or Indian oceans. The Arctic Ocean, the Mediterranean and Caribbean seas, and the Gulf of Mexico are considered marginal seas of the Atlantic Ocean. These, in turn, have their own marginal bays and seas. Narrow, shallow straits separate the marginal seas from the Atlantic: the Straits of Florida (Gulf of Mexico), the Strait of Gibraltar (Mediterranean), and many gaps between the islands of the Greater and Lesser Antilles (Caribbean Sea).
Many other large bodies of water have been designated as seas, but all are marginal to the great oceans. The largest of these are the Bering Sea, Coral Sea, East China Sea and South China Sea, Sea of Japan, Sea of Okhotsk, Yellow Sea, and Philippine Sea, bordering the Pacific; the Arabian Sea, Red Sea, and Bay of Bengal, bordering the Indian Ocean; and the Scotia Sea, North Sea, Labrador Sea, Weddell Sea, Norwegian Sea, and Greenland Sea, bordering the Atlantic Ocean. Marginal seas differ from the major oceans primarily in size, but also in depth and bottom topography.
The boundaries between the oceans are based on geographic criteria and have little to do with physical water-mass boundaries. The Atlantic is separated from the Indian Ocean by the 20° east meridian. It is separated from the Pacific Ocean, in the south, by a line extending from Cape Horn at the tip of South America to the South Shetland Islands off Antarctica's tip; and, in the north, by the narrowest part of the Bering Strait. The dividing line between the Pacific and Indian oceans extends along an arc through the Malay Peninsula, Sumatra, Java, and Timor to Cape Londonderry in Australia; to Tasmania; and then along the 147° east meridian to Antarctica.
Reference is sometimes made to the Antarctic, or Southern, Ocean. This encircles the Antarctic continent and consists of the southernmost sectors of the three principal oceans. Despite the lack of definitive geographic boundaries, the meteorological and oceanographic conditions in the high southern latitudes combine to produce a well-defined circumpolar current called the West Wind Drift. Any reference to an Antarctic Ocean refers roughly to the waters bound by this current; otherwise, as noted, the Atlantic, Pacific, and Indian oceans are considered to extend down to Antarctica.
Oceanic regions constitute a much larger percentage of the Earth's surface in the Southern Hemisphere (81%) than in the Northern Hemisphere (61%). This factor is reflected by major differences in oceanic circulation and weather patterns between the two hemispheres.