alt Hacker News> My take would be that underwater temps cannot get below freezing
Yep, density of water changes with temperature—it's the only thing that has that phenomenon—and therefore the colder water is always at the surface. That's actually what made life possible since life could survive despite a frozen surface.
Happy to be proven wrong, but that's what I learnt making cocktails and investigating clear ice :)
Lots of things change density when temperature changes. Water is unique that below 4°C, it gets less dense as it cools - and that trend continues even through the phase change. That's why, as you mention, ice floats, and the surface freezes first, and fish can live below a frozen surface.