[UKönig]

Thermal layers...
Used first in SH1, with the US vs the Japanese in the Pacific.
Thing about the Pacific ocean is that it's the largest ocean on the planet, with the longest dwell time for the sun to heat everything up.
It is also at the same time where you can find both the most shallow, and ridiculously deep seas.
Thermal layers will generally appear in the Pacific in the areas where your fleet submarine will operate against Japanese shipping.
But the Atlantic is brutally cold, most of the time.
Especially the north Atlantic.
Uboats were typically found in the colder seas, at least at first, with the Baltic, (grids AO-AG)where the training was done, the North Sea, between the west coast of Germany, including at the AN and AF grids, the west coast of Scotland and England.
Getting out to deeper waters in the mid altantic, there really isn't anywhere to find warmer water.
The only conceivable place for thermal layers in SH3 would be in the Mediterranean off the eastern seaboard of the United States, from cape Hatteras down to the coast of Florida.
Of course, the Caribbean Sea is going to be much warmer, as the south coast of Africa, but Doenitz never really intended for the uboats to be deployed in those areas, so the colder north Atlantic with its general lack of thermal changes were what got modelled in the game.
If you play it out as they (the programmers) want you to, then you should pretty well never find a thermal layer anyway, so why bother including that extra bit of processing data to the game?
Sure realism is nice, but the decision to rely on submarine profile only was probably the best way to go.