Unlike fresh water, which freezes at 32 degrees Fahrenheit, sea water freezes at 28.4 degrees Fahrenheit. The 3.6-degree difference in freezing temperature is due to the salt that is found in sea water, making it slightly more difficult to freeze.
Despite its lower freezing temperature, at least 15 percent of the ocean is covered by ice at some point in the year. The average temperature of the ocean is 38.3 degrees Fahrenheit, only 9.9 degrees above freezing. Salinity or saltiness is a major determination factor of the exact freezing temperature. Sea water has a typical salinity of 35 parts per thousand, which makes its freezing temperature lower than that of fresh water.