Measure the salinity of the water. Use a salinometer, which measures the electrical conductivity of your sample. It will calculate the salinity on its own and give you a readout. Generally, sea water has a salinity of 35 parts per thousand (ppt).
Calculate the temperature at which the water will freeze. Sea water with a salinity of 35 ppt freezes at about -2 degrees Celsius. The progression is linear, so if the water had a salinity of 70 ppt, for example, it would freeze at -4 degrees Celsius; or at a salinity of 17 ppt, it would freeze around -1 degree Celsius.
Check the roughness of the water. If the area is constantly thrashing with waves, it will take longer for the water to freeze, no matter what the temperature or salinity.
