According to National Snow & Ice Data Center, saltwater has a relatively lower freezing point than freshwater, which freezes at 0 degree Celsius. And for saltwater, its freezing point decreases by 0.28 degree Celsius for every 5 ppt increase in salinity.
The graph above shows the freezing point of salt with respect to its salinity. By adding certain portion of salt into fresh water, the freezing point continuously decreases till around -21 degree Celsius when the salinity is around 290g/kg. After this point the freezing point begins to increase as the salinity increases.