In real life 1939 to 1945, no German submarine was fitted with a device to monitor external water temperture.
Why?
At that point in history very little/nothing was understood about thermal layers and the effects these had on ASW warfare.
By diving deep U boat Kaleuns exploited thermal layers, but had no idea they were doing it. All they understood at that time, was that "safe", meant going deeper. Thereby accidently encountering thermal layers of which they had no knowledge of.
Hope this helps.
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Sir Humphey Appleby, GCB, KBE, MVO and MA. Britain's Greatest Orator, well bar that Churchill fellow.
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