I must have read this back in the '80s when I was working my way through a number of such works. A least one of the protagonists is a U-boat captain who turns against the Nazis. The one scene I remember vividly is where he freaks out when he learns the felt boots issued to the Kriegsmarine were made from the hair of people sent to extermination camps. He then tries to rid himself of the shame of wearing them by bathing repeatedly in ice-cold water.
Charming story, eh? I’m pretty sure it wasn’t Das Boot because I don’t recall ever reading that one.