Anything dissolved in water will depress its freezing point. The compound's efficacy at depressing the freezing point depends on a few things; generally, the lighter the molecule and the more it ionises (divides into smaller fragments) when it dissolves, the more effectively it will reduce the freezing point.
Sodium chloride is a pretty light molecule and it splits into two ions when it dissolves, so it's fairly good. 1% of sodium chloride reduces the freezing point by 0.576 degrees centigrade, and you can scale that relationship up/down,
i.e.
pure water (0% sodium chloride) freezes at 0 degrees C
1% solution of sodium chloride in water freezes at -0.576 degrees C
2% solution of sodium chloride in water freezes at -1.152 degrees C
0.5% solution of sodium chloride in water freezes at -0.288 degrees C
etc.
So if the prevailing temperature outside is -7 degrees (for example), you will need more than 12.2% (calculated as 7/0.576) of sodium chloride dissolved in order to prevent water from freezing.
The other issue, of course, is getting the salt to dissolve! The lower the temperature, the more difficult that is. It's obviously easier to have salt as a preventive measure to stop water from freezing, rather than trying to treat water that's already frozen.
Assuming sea water has a typical salt concentration of 3.5%, it would freeze at just under -2 degrees centigrade (calculated as -0.576 x 3.5). But the sea would actually need to be even colder than that in order to freeze hard because the motion of the waves militates against freezing.
Here's seawater just on the point of freezing.
As I said, anything that dissolves in water can depress its freezing point, but they vary in how effective they are. For example, table sugar (sucrose) reduces water's freezing point by only 0.047 degrees for every 1% of sucrose. Therefore you'd need a theoretical concentration of 148% sucrose (which is obviously impossible!) to get water to stay liquid at -7 degrees.
Incidentally, just like dissolving anything in water lowers its freezing point, so too it raises its boiling point. That's why salt is added to water when cooking pasta
etc. It's not just a flavour thing - it actually allows the liquid to reach a higher temperature. If you were just boiling food in plain water, it could only go to a maximum of 100 degrees (then it evaporates), whereas with salt (or anything else) dissolved in it, it can be brought to a higher temperature to cook food faster. Just as with the freezing point depression, the extent to which the boiling point is elevated depends on the concentration of what's dissolved (the higher the concentration, the higher the boiling point) and the nature of what's dissolved.