If the crystalline structure of water did not expand from it's liquid phase (thus lowering it's density) when it freezes, then ice would not float. Ice would sink trapping everything below it. It's theorized that this little fact means life likely couldn't exist if ice didn't behave this way.

As water starts to freeze the molecules start to rearrange thrmselves from a free flowing random motion into a crystal lattice. As it does this the molecules push each other a little further apart and lock into place, but because they are further apart they now take up more 'space' and as such they are less dense. The water wround them now has more water molecules for the same volume that the ice does and that means the water sinks (as its heavier) and ice rises.

To really ELI5: Imagine water is like a crowd of people balanced on a REALLY giant see-saw, everybody is really squished together and when they get cold they decide to put their arms out wide and grab hands with the people around them, but making small circles with the other people also grabbing hands until there is a big group of people made of little rings of people arms out holding hands. Now imagine that the people with their arms out have to spead out and some fall off the edge of the see-saw. Now there are less people on one side of the see-saw and because the other side is heavier, that side drops and the people with their arms out rises.

Ice doesn't expand. Ice as a solid shrinks when getting colder. H2O expands during phase transformation from liquid to solid.

Thank you... I'm an idiot and wasn't thinking about the Southern Ocean as in 'The' Southern Ocean... instead I was thinking about it in terms of the oceans in the south (as in, not necessarily glacial). But yes, now I get it and understand. It probably would have helped if I had opened the link to start with.

Is that the one where the Latinos have mutated and are heating up the planet?

Oh yeah I didn't think about that one. 2012 was a fun bad movie. Some of Emrich's best. And based off an extraordinarily weird book by a guy Einstein thought was a genius.