[Phaedrus]

As far as I have read:

Thermal layers were not much of a factor in the U-boat war, or at least - u-boat operations in the North Atlantic ocean.

Generally, the colder the water, the shallower the highest temperature level of thermocline.
The technique of "hiding under the layer" is usefull at the weakest end of the Asdic / Sonar beam, but in cold northern ocean water, there would be a significant temperature difference between the surface water down to about 10 meters (Say 5 - 10 degrees), but the water following 10 m (or much, much less depth in rough seas) would remain uniform due to the mixing of warmer shallow water with deeper ocean water by waves and current. The next significant temperature change at around 230-260 might be as much as 20 degrees - but generally deeper than the operating depth of the u-boat.

Tropical Atlantic water might see water at 24 degrees dropping down to 22 over 100 m, but with a temperature change from 22 to 12 degrees going to 200.

Hope this sheds a little light on the layer issue!