In Das Boot the crew tries to dive through the sea near Gibraltar, although they know that's very risky.
Why do they do that?
What they try to achieve in the Mediterranean Sea?
The Medditeranian sea was very much a region under control of the allies from 1941 onwards with the Italian Navy suffering big losses and the allies controlling Gibraltar as the 'gate'. The Germans sent 62 U-Boats into the Mediterranean despite the risks in order to limit the overwhelming control that the allies had there.
However it was a very risky arena for the U-Boats, calmer water, allied air-cover with radar, and they had to get past Gibraltar. No U-Boats ever made it back from the Mediterranean.
It was not a universally popular policy in the German military, Donitz (Commander of the submarine fleet before being elevated to Grand Admiral) considered it wasteful of resources as each U-Boat sent into the Med was no longer ever available in the Atlantic.