1) Das Boot, as others have said, extended edition. It’s in German but with English subtitles. It is a phenomenal movie and doesn’t get enough mainstream attention in the WWII cinematic cannon.
2) Hunt For Red October for sure, it’s got the great acting and writing (albeit unrealistic submarine operational depiction). Still good
3) Down Periscope - this is a comedy. Many submariners like to say this is the most realistic submarine movie (to which I roll my eyes) but it is true that it’s the best movie about submariners.
4) Run Silent Run Deep: old Clark Gable movie about US submarine operations against Japan. I’m dying for a modern remake. Very good film
5) U-571 - what this gets right is that all submarine officers are hot
I hate the most recent submarine movie Hunter Killer with Gerard butler, but I almost like to hate watch it.