Sea water freezes at 28.4 degrees Fahrenheit. Fresh water freezes at 32 degrees Fahrenheit. Water that is a saturated salt solution, which has as much salt as the water can hold, freezes at minus 6 degrees Fahrenheit.
The salt content of the ice is almost non-existent, making it drinkable once melted. While it is the salt content that causes the lower temperature for freezing, the salt does not freeze. As the salt molecules move in response to the temperature change, the water molecules freeze into crystals around them. The salt can then be wiped away before the ice melts, making the result salt free.