While pure water freezes at 0°C (32°F), salt water needs to be colder before it freezes and so it usually takes longer to freeze. The more salt in the water, the lower the freezing point.

Very salty water freezes at around -21 °C, or about -6 °F. What is interesting is that this effect is used all over the place for practical reasons. Often, salt is put on roads to melt ice. If there's a lot of ice, you need a lot of salt.

So then the question is why does salt water need to be colder to freeze than fresh water? Basically, ice is a crystal, an almost perfect array of pure water molecules with almost no salt in it. To make that out of pure water requires limiting the ways the water molecules move around. To make that out of salt water requires BOTH limiting the ways the water molecules move around AND limiting the ways the salt can move around (it's stuck in the liquid, or in separate crystals), which is harder to do.