Oceans

technically, those bodies of water that occupy the ocean basins, which begin at the edge of the continental shelf. Marginal seas such as the Mediterranean, Carribean and Baltic are not classed as oceans. A more general definition is all the water on the Earth's surface, excluding lakes and inland seas. The oceans are the North and South Atlantic; North and South Pacific; Indian and Arctic. Together with all the seas the salt water covers almost 71% of the Earth's surface.

From the shore the land dips away gently in most cases--the continental shelf--after which the gradient increases on the continental slope leading to the deep sea platform (at about 4 km depth). There are many areas of shallow seas on the continental shelf (epicontinental seas) e.g. North Sea, Baltic and Hudson Bay. In the ice age, much of the shelf would have been land and conversely should much ice melt, the continents would be submerged further. The floors of the oceans display both mountains, in the form of the mid-oceanic ridges, and deep trenches. The ridges rise 2-3 km from the floor and extend for thousands of kilometres while the trenches reach over 11 km below sea level, at their deepest (Mariana Trench, south-east of Japan).

The oceans contain currents, i.e. faster-moving large-scale flows (the slower movements are called drifts). Several factors contribute to the formation of currents, including the rotation of the Earth, prevailing winds, differences in temperature and sea water densities. Major currents move clockwise in the northern hemisphere and anticlockwise in the southern hemisphere. Well-known currents include the Gulf Stream and the Humboldt current.

Taken from Dictionary of Science

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An ocean (from Greek Ωκεανός, Okeanos (Oceanus)) is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (~3.61 X 10¹⁴ m²) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.

More than half of this area is over 3,000 metres (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Scientists estimate that 230,000 marine life forms of all types are currently known, but the total could be up to 10 times that number.

Taken from Wikipedia