[UKönig]

I've read that by 1943, early 1944, when the odds of a German victory were getting daily, smaller, the French "co-operation" began to break down. Sabotage could also include canned food of the 'lowest quality', and/or a dead dog in the fresh water tank. Anything to incapacitate the U-boat crews.
Even before boats could set sail, a German diver had to inspect the hull for limpet mines that may have been placed by the french resistance or loyal dockworkers. By 1944, just because you made it back to base, did not mean that you were safe...