The freezing temperature of salt water depends on how much salt you put in. Oceanographers define the “salinity” of water as the number of grams of salt per 1000 grams of water. (Since 1000 grams of water is almost exactly 1 liter, it’s also grams of salt per liter).
Here’s a table which gives the freezing point of water at a number of different salinities:
S(g/kg) 0 10 20 24.7 30 35
T(freezing) (C) 0 -0.5 -1.08 -1.33 -1.63 -1.91
Away from rivers and glaciers, the ocean has a salinity of about 35. The Atlantic is about 1 salinity units saltier than the Pacific. So ocean water freezes at about -1.91 degrees C.
Expect the freezing temperature to drop by .054 degrees for each gram of salt.
When saltwater freezes, the salt can’t join with the ice crystals, and tends to stay in the liquid water. As a result, ice made from salt water has less salt in it than the water that’s left behind. The unfrozen water gets saltier and saltier, and eventually becomes the solution will become saturated. This happens for salt in water at -21.1°C, which therefore is the coldest a saturated solution of salt and water can get.
At that temperature, the salt begins to crystallize out of solution, along with the ice, until the solution completely freezes. The frozen solution is a mixture of separate salt (NaCl·2H2O) crystals and ice crystals. This heterogeneous mixture is called a eutectic mixture.