Thermodynamics is apparently a little-studied branch of physics.
At least by these pseudo-scientists that claim to be authorities on Global Warming.
All other things being equal, warmer water is above colder water. The degree of salinity would change that (very saline water is heavier than very cold water that is less saline), but of equal salinity, the warm waters would be at the top, the cooler waters would be at depth.
About 1,500 meters of depth, water almost everywhere in the ocean is at a steady 4 degrees Celsius. Doesn’t matter if it is at the equator, or under the polar ice caps. Water is most dense at that temperature, and in fact, has to EXPAND to freeze. That is why ice floats.
Thermodynamics is HARD - maybe you are good at it, but it was a struggle for me ... just my $.02 ..