(An ocean is a large area of water between continents. Oceans are very big and they join smaller seas together. Together, the oceans are like one "ocean", because all the "oceans" are joined. Oceans (or marine biomes) cover 72% of our planet.[1] 
The largest ocean is the Pacific Ocean. It covers 1/3 (one third) of the Earth's surface. Big and small fish of different types live in oceans. Crabs, starfish, sharks, whales etc are also found in oceans.)

The smallest ocean is the Arctic Ocean.
Different water movements separate the Southern Ocean from the Atlantic, Pacific and Indian Oceans. The Southern ocean is also called the Antarctic Ocean, because it covers the area around Antarctica. Older maps may not use the names Arctic Ocean and Southern Ocean.

The deepest ocean is the Pacific ocean. The deepest point is the Mariana Trench, being about 11,000 metres (36,200 feet) deep. The deep ocean is characterized by cold temperatures, high pressure, and complete darkness. Some very unusual organisms live in this part of the ocean. They do not require energy from the sun to survive, because they use chemicals  from deep inside the Earth (see hydrothermal vent).

Although many people believe that the oceans are blue because the water reflects the blue sky, this is actually not true.  Water has a very slight blue color that can only be seen when there is a lot of water. However, the main cause of the blue or blue/green color of the oceans is that water absorbs the red part of the incoming light, and reflects the green and blue part of the light.  We then see the reflected light as the color of water.

Organisms that live in oceans can live in salt water. They are affected by sunlight, temperature, water pressure, and water movement. Different ocean organisms live near the surface, in shallow waters, and in deep waters. Small plant organisms that live near the surface and use sunlight to produce food are called phytoplankton. Almost all animals in the ocean depend directly or indirectly on these plants. In shallow water, you may find lobsters and crabs. In deeper water, marine animals of many different shapes and sizes swim through the ocean. These include many types of fish, such as tuna, swordfish and marine mammals like dolphins and whales. The skies above the open ocean are home to large sea birds, such as the albatross.[1]

Nations like Russia and Japan have lots of huge ships that go to some of the world's best fishing areas for many months. These large ships have libraries, hospitals, schools, repair (fixing) shops and other things that are needed for fishermen and their families.

Many people look at the sea as a source of food, minerals and energy.

According to the FishBase.org website, there are 33,200 known species of fish, and many of them live in the oceans.[2]  Many of these fish are a fine source of protein, so many people eat them. Fishing industries are very important because they make jobs and give food to millions of people. Today, usually through ocean fishing, the ocean supplies about 2% of the calories needed by people.[3] Tuna, anchovies, and herring are harvested close to the surface of the ocean.[3] Pollock, flounder and cod are caught near the ocean floor.[3] More than a million tons of herring are caught every year in the North Pacific and North Atlantic, and almost eight fish out of ten fish are eaten as food for humans. The other fish are used as fertilizer, glue, and pet and other animal food.[3]

There are many different ocean temperatures in the open ocean, both vertically (from top to bottom) and horizontally. Icebergs are made over very cold waters at either pole, while waters at the equator are pretty warm.[3] Water cools and warms more slowly than land does, so land influenced by the ocean has later and milder seasons than land that is farther away from the ocean.

The surface part of the ocean, also called the mixed layer, is not much colder even when we go deeper down.[3] Below this surface zone is a layer of sudden temperature difference, called a thermocline. This is a middle layer hat is from the surface zone down to about 2,600 feet (800 m). Thermoclines may happen only at seasons or permanently, and may change depending on where and how deep it is. As evaporation happens, it begins cooling, and if the water evaporates very quickly, the water becomes saltier. The salty, cold water is denser, so it sinks. This is why warm and cold waters do not easily mix. Most animals and plants live in the warm upper layer. Below the thermocline, temperature in the deep zone is so cold it is just above freezing - between 32–37.4 °F (0–3 °C).[3]