Why the water in the Mariana Trench is warmer (34-39 F) than the water the victims of the Titanic fell in (28 F)?
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Water is densest at about 4 degrees C (upper 30s F). Thus, that is the temperature that water at the very bottom of any water body will tend toward. Salinity does have some effect, but if salinity remains constant, the densest water is about there, at a temperature a few degrees about freezing.
Water is a weird substance. Its solid version (ice) is less dense than its liquid version. Most substances constantly increase in density as temperature drops. Water does not. However, the good news is that this is why lakes do not freeze solid. The ice floats. It is also why most lakes have warm water at surface but a cool deep layer.
So, the question here is really “why is the water so cold in the region of the Titanic?” It should not be if what I just said is true. The reason for this is that the deep water around the Titanic is coming from up north, in and near the Arctic Circle, and is kept at ice-temperatures (very near freezing, even a little lower because it is salt water and salt water freezes below 0C/32 F). It would “like” to go straight south along the surface, but the waters of the Gulf Stream are also trying to head north along that same route, more or less. The cold water from the polar region is denser than the warmer water from the equatorial region, so the colder water sinks under the warmer water (has to go somewhere and down is its best choice).
There is also a bit of a salinity enrichment thing happening, where the freezing out of pure ice makes the cold ocean get saltier, and thus more dense than average sea water. The salinity enrichment factor is more important during the cold season, of course.
The end result is that the deep water of the North Atlantic is very cold, unusually cold. This sinking of cold saline water is actually part of the driving of ocean current motion, and climate scientists worry that warming will release too much fresh water (from melted ice) and shut down that deep current, which would also shut down the Gulf Stream, and the north Atlantic would become a source of cold for western Europe, rather than a source of warmth like it is now.
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