I don't think I have seen another movie that made me feel so much 'part of the experience' as 'Das Boot' does.
'Hunt for Red October' is a great movie. But it's not thát much about the submariners and life on board, specialy in time of war.. But very good nonetheless.
For all it's flaws, 'U-571' was pretty entertaining.. though far fetched here and there.
Another one I recently watched again is 'Below', which is a good one too. But due to it's genre, not too realistic.
I liked 'Crimson Tide' a lot aswell. But again, not as much about 'heartship in wartime' as Das Boot, for example. And most of these 'modern sub' movies do not show an accurate submarine interior, I think. The lighting is all wrong too if you ask me. Not that I am an expert, but they also seem to have a LOT of unused space to spare in those movies sometimes..