[Cybermat47]

Update for anyone wondering the same question; it looks very likely that U-75 went through the canal on her April 10th - May 12th 1941 patrol. She departed from Kiel and, according to uboat.net, was in the position shown in the first attachment three days into her patrol. But for U-96’s first patrol in December 1940, she went to the Atlantic through the Baltic route.

So, it seems that u-boats went through the canal at certain periods, and through the Baltic route in others.