Obviously all of the ocean is above freezing; but reading about the titanic sub today, I still was surprised to read that it’s still above freezing down 2+ miles down. What keeps it relatively warm? Obviously there is some volcanic activity but it’s not widespread over the entire ocean is it?
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Lots of wrong answers here. The short answer: things only change their temperature if they are heated or cooled. The deep ocean has nothing to heat it up and nothing to cool it down, so it stays at the same temperature.
Longer answer: the earth’s oceans are slowly overturning. Cold water near the poles is denser than than warm water, so it sinks and spreads out along the bottom. Water near the surface in the rest of the world is warmed and rises. This creates an overturning circulation, a “conveyor belt” in which water cools at the poles, sinks, flows toward the tropics, rises, and returns to the poles.
Thus, the water you find at the sea floor has kept the same cold temperature it had when it was last in contact with the atmosphere, in the Antarctic or Arctic, thousands of years ago.
The bottom water is heated by geothermal heat from the sea floor, but only a little bit.
Other responders have mentioned that water has a maximum density at a temperature of 4 C. This is true for freshwater, but seawater does not behave this way. Seawater just get denser until it freezes.
https://en.m.wikipedia.org/wiki/Thermohaline_circulation
https://en.m.wikipedia.org/wiki/Seawater#/media/File%3AWaterDensitySalinity.png
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