The ocean covers nearly 71% of the Earth's surface and is divided into major oceans and smaller seas. The three principal oceans, the Pacific, Atlantic, and Indian, are largely delimited by land and submarine topographic boundaries. All are connected to what is sometimes called the Southern Ocean, the waters encircling Antarctica. Important marginal seas, primarily in the Northern Hemisphere, are partially enclosed by landmasses or island arcs.

The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria. These divisions are (in descending order of size):

• Pacific Ocean, which separates Asia and Australia from the Americas
• Atlantic Ocean, which separates the Americas from Eurasia and Africa
• Indian Ocean, which washes upon southern Asia and separates Africa and Australia
• Antarctic Ocean, sometimes considered an extension of the Pacific, Atlantic and Indian Oceans, which encircles Antarctica.
• Arctic Ocean, sometimes considered a sea of the Atlantic, which covers much of the Arctic and washes upon northern North America and Eurasia.