Water

Water is a common chemical substance that is essential for the survival of all known forms of life. It is found mostly in oceans and other large water bodies, with saltwater oceans holding 97% of surface water. Water moves continually through a cycle of evaporation or transpiration, precipitation, and runoff, usually reaching the sea. Clean, fresh drinking water is essential to human and other life.

Oceans

An ocean is a major body of saline water, and a principal component of the hydrosphere. While all oceans comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean, five “separated” oceans are usually recognized: Pacific Ocean , Atlantic Ocean , Indian Ocean , Southern Ocean and Arctic Ocean

Aquatic life forms

The earliest life forms appeared in water; nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales that also live in the water. Aquatic animals must obtain oxygen to survive. Fish have gills instead of lungs. Marine mammals need to surface periodically to breathe air. Smaller life forms are able to absorb oxygen through their skin