It was an absolute given that Das Boot would by far be the first movie mentioned by so many of them in this post. It was the instant first movie that came to my mind and nothing comes quite close, even today, decades later. An absolute masterpiece of serious film making.

Fun facts now: the cast of Das Boot was obliged to constantly stay indoors during the entire length of filming, forcefully as part of an effort to both give them a grossly pallid, sickly complexion like you'd expect from a WWII era submariner at sea for months, and to create a sense of claustrophobia that would percolate into more realistic acting.

Also, the mockup of the submarine's interior was built for maximum realism in its size and all usable physical details, with the actors rigorously trained to move through this space as naturally as possible (as a real German U-boat crew would)

The effort, along with the great script, fantastic cast and of course, memorable music, shows in pretty much everything, right down to the disgusting details of how they look and act after weeks at sea. One hell of a movie, and while my personal experience with submarines is zero, this is the one that feels like it should be absolute most realistic depiction of crewing a sub from that era. It fully deserves its rank as one of the most highly rated films of any kind on most movie ranking websites, like IMDB and etc.

I can also see why it's the most highly voted film among submariners. Even if modern nuclear subs are at a whole other level of comfort compared to anything from 85 years ago, certain basics stay the same: It's a claustrophobic, fully enclosed space with nothing but artificial lighting, observation almost entirely through instruments, and crushing, nearly inescapable, horrible death just a few inches of hull and a couple sudden mistakes away.

All this is the case in a way that just doesn't apply to the same degree with any surface vessel, where you can still somehow feel directly connected to and within reach of the wider, comforting world.

It probably has more in common with space travel than being out at sea in these characteristics.

Even if authentic, a modern nuclear sub movie just isn't going to have the same feel. I suppose you could cite K-19: The Widowmaker though that's relatively old at this point as well.