Salinity is usually measured in parts per thousand or ppt . Percent is parts per hundred. The chloride ion mass is proportional to the salinity, and chloride was commonly measured in the past and converted to salinity. There are also many minor constituents in seawater. Sea Water Density a.
Density is affected by temperature, salinity, and pressure.
However, in most cases seawater is considered incompressible, so not greatly affected by pressure. Density is a very important property of water masses since it determines whether they sink or float. In a stable ocean density increases with depth. This depresses the temperature of maximum density down to -1.33 o C at a salinity of 24.7 ppt, which is also the initial freezing point at this salinity. This explains why seawater seldom freezes except near the Poles - as it cools it continues to sink.