Ocean water is a complex mixture of gases (air) and dissolved solids (salts, especially sodium chloride). Marine organisms are dependent on dissolved gases for survival. Salinity is the salt content of a body of water. It is the main mineral of ocean water and is also called sodium chloride (NaCl). The salinity of ocean water is caused by dissolved mineral compounds that come from the weathering of rocks. The salinity of ocean water varies in some places depending on rates of evaporation and amount of runoff from nearby land. Ocean water is denser than fresh water because of its salinity.
Water pressure is the weight of the water pressing on an object. Water pressure is greater than air pressure because water weighs more than air.