The density of seawater varies with temperature and salinity of the water. As temperature increases, density decreases. As salinity of the water increases, density increases too. In order to determine, what affects the density of seawater more it is necessary at first to produce an average data for density and salinity of seawater. A good estimate of its density at the ocean’s surface is 1025 kilogram per cubic meter. On average, seawater in the world’s oceans has a salinity of ~3.5% (every 1kg of seawater has approximately 35 grams of salts, mostly, but not entirely, sodium chloride dissolved in it). The typical temperature of the sea (ocean) is 18 Celsius degrees.
At the figure below the density of the seawater dependent on the temperature and the salinity is presented. At the x-axis the salinity is presented in the grams of salts per 1 kg of sea water. At the y-axis the temperature of seawater is presented in Celsius degrees. The diagonal lines show the sea water density with value, labeled on the lines. The values of seawater density, shown on the figure do not include the density of fresh water: 1000kg/m3. Hence, the label 25 on the line shows the destiny 1025kg/m3. The black point shows the typical seawater. The vast majority of seawater has a salinity of between 3.1% and 3.8%. (The figure presented below shows a little bit narrowed region)