Composition of sea water? Sodium 30.4%, 3.7% magnesium, calcium 1.2%, 1.1% potassium, 0.04% strontium, chlorine 55.2%, 7.7% sulfate, 1.19% bromine, carbonate and bicarbonate 0.35%, 0.07% boric acid. The water density It can vary according to the salinity and temperature. The medium value used in naval architecture is 1.025Kg m³