The Enemy Below (1957) - Watched this last night, there were definitely similarities with the later Das Boot and even The Hunt for Red October. Das Boot is way more grim than Enemy Below, but this doesn't take away from the excitement of the final battle scene in the latter. One thing I did not understand was why the German captain was wearing what looked more like an unbuttoned fancy silk shirt than a buttoned naval shirt, and then later in the final scene he again did not button what did look more like a uniform shirt. Especially for a captain I thought this looked sloppy and unprofessional, but that's a minor quibble.
Interestingly there were two different versions of the ending, the test audience unequivocally chose the happier ending. I would have preferred the unhappier ending, but that's just me.
Anyway, thanks to Nomad for mentioning this movie, I'm glad I saw it.
Speaking of submarine movies I think the worst I've seen were K-19: The Widowmaker (2002) and U-571 (2000). Don't remember exactly why I thought they were stinkers, I just remember not liking them.