A ocean is a large area of saltwater 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. 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 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.