Key Difference: Oceans are large bodies of water that exist on the Earth. A sea is a large body of saline water that may or may not be connected to an ocean.

Oceans and seas are both bodies of water and these terms are often used interchangeably due to their lack of proper definition.  Water bodies make up the hydrosphere that covers 71% of the Earth. Oceans and seas have slight differences among them.

Oceans are large bodies of water that exist on the Earth. An ocean is a body of saline water that composes a large part of a planet's hydrosphere. Though for many years there were only four known oceans, in 2000, the International Hydrographic Organization added the Southern Ocean to the list. The five oceans in descending order of area are the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans. Approximately72% of the Earth is covered by water and though there are various different oceans, the waters comprise one global, interconnected body of salt water sometimes referred to as the World Ocean or global ocean. There is no official description for an ocean, except for a vague definition stating a vast body of saline water. Oceans are the largest body of water on the planet and contain approximately 230,000 types of marine life. The depth of an ocean ranges between anywhere from 13,000 feet to 35,000 feet.

Oceanographers divide the ocean into different zones depending on the present physical and biological conditions. The oceans are further categorized into zones by depth, light and other factors. The various different zones include pelagic zone (all open oceans regions), photic zone (surface to 200 meters down) and aphotic zone (depths exceeding 200 meters). The depth of an ocean ranges from 20,000 feet to 36,000 feet. The ocean currents also play a bit part in the Earth climate, by transferring heat from the tropics to the polar regions, and transferring warm or cold air and precipitation to coastal regions, where winds may carry them inland.

A sea is a large body of saline water that may or may not be connected to an ocean, though commonly these are. However, the body is smaller compared to an ocean. It is believed to lack a natural outlet. Seas are usually located near land and are used to connect the land with an ocean. It is defined as, “a division of an ocean or a large body of salt water partially enclosed by land.” Often seas and oceans are used interchangeably due to the fact that they are connected.

Seas are also connected to the land and have turn into beaches. The seas are also associated with heavy waves that do are not visible in the oceans. The term ‘sea’ is derived from a pure Germanic word. Well known seas include the Mediterranean, Baltic, Bering, Black, Caribbean, Coral, North, Red, and Yellow. The largest seas are the South China Sea, the Caribbean Sea, and the Mediterranean Sea.

Image Courtesy: treehugger.com, stuffpoint.com