The freezing point of seawater depends upon it's salinity,
which is the amount of salt that it contains. Open ocean seawater has a salinity of about 35 (no units are used for salinity anymore, although you may see it called o/oo which means parts per thousand or psu, which means practical salinity units. Neither usage is now considered correct). Anyway, fresh water freezes at 0 degrees celsius and 35 water freezes at about -2 degrees C. The decrease is linear so that water with a salinity of 17 freezes at about -1 degree C.