How cold would it have to be for the entire ocean to freeze?

Are we talking about the entire surface of the oceans? Or literally all of the water in the oceans? Either way this is a somewhat complicated scenario for a few reasons. We know that “pure” freshwater freezes at 0 degrees Celsius. read more

Ocean water will freeze just like freshwater, but at lower temperatures. Seawater freezes at about 28.4 degrees instead of 32 because of the salt in it. read more

To freeze the surface of the whole ocean would take a global catastrophe. It's unlikely that the deepest parts of the ocean would freeze for a long long time due to pressure (if ever on a billion year time scale). read more

Yes, the entire planet can freeze. It has happened at least once in Earth's history. Life at that time was all (maybe almost all) mono-cellular. We don't know how it survived - around volcanic vents is a decent guess, or perhaps the global freeze was short enough that the oceans diddn't freeze all the way down. read more