Here is a wonderful read about sound and how it relates to the ocean:
http://www.fas.org/man/dod-101/navy/docs/fun/part08.htm
"There are many areas in the North Sea and, even worse, the Baltic Sea!!, where a submariner couldn't ever expect any comfort from Thermoclines."
Pg 29 in the
U-Boat Commander's Handbook states: "In the Baltic and North Sea, conditions are generally bad for sound locating; in other words, favorable for submarines"
I'll read up a little more from two other sources but I want to say the boot had a way to check temperature during that timeframe. I may be way off but I'll check back in a bit.