How cold would the sea have to be to freeze?

Pure water contains no impurities and freezes at 0°C. However, seawater – on average – contains 3.5 per cent of dissolved salts, which lowers the freezing point; this phenomenon is called freezing point depression. Freezing point depression is a colligative property of matter, which means it depends on the number of molecules present, not on the specific type of molecule or their mass (i.e. the dissolved substance doesn’t have to be salt, but any soluble substance, it’s the amount of dissolved molecules that is important). In terms of seawater, the average freezing point is about -2°C. However, it does vary around the world due to differing amounts of chemicals (e.g. salts) dissolved.