If you liked Das Boot, you might also like...

There's an old 1950's tv show called The Silent Service, which is good for its time. It gets a bit repetitive if you binge, but some good stories.

Should note that my grandfather was asked to submit his wartime story/script to the show. He wrote up his experiences (was in Manila on Dec 7), but ultimately they wanted to make too many changes, elaborations. He was a stickler for the truth in that way.

You can find it here. https://chuckosborne.wordpress.com/

I'm surprised no one has mentioned "Murphy's Law" (1971) as a German U-boat & crew is instrumental to the plot.

Another recommendation -- don't laugh -- is "The Land that Time Forgot" (1974) where again, the story involves a German U-boat, albeit from WWI. It's actually a well done fantasy adventure, just ignore the cheesy puppet dinosaurs or roll with their quaintness.