There are other salts dissolved in sea water, with ordinary sodium chloride constituting about 90% of the dissolved salt. The table at left is given by the Windows on the Universe website. It is not clear how you reach charge neutrality with these numbers. Another item of information from this site was that deep ocean water was at about 3°C and salinity 34-35 ppt. Seawater of average salinity 35 ppt freezes at -1.94°C (28.5°F). The salinity of the water is measured with a CTD instrument (Conductivity, Temperature, Depth). The conductivity is used to calculate the salinity in conjunction with the temperature and depth (pressure) measurements.