As for World War I, I can't believe I didn't think of this one sooner: Lusitania: Murder on the Atlantic. I take issue with the title (it was toned down for subsequent releases), but it's still an outstanding film overall, and includes a number of scenes aboard U-20. If you have a choice, watch the British version; the American release on A&E was cut for time and profanity.
One caveat: They used the U-boat from Das Boot, so it's not a historically accurate 1915 sub.
Specifically, they used the SET from
Das Boot. It was a full-blown, correct version of the complete internals of the type VIIC built on a gimbal. Apart from that,
Das Boot featured a full-sized conning tower in the studio, a full-sized replica of the hull (without internals and power, but built by the original shipyard), an RC miniature (using standard Ken and Barbie dolls, dressed by the costume department, in the conning tower if needed. The Barbies were cross-dressed and standing in for shorter cast members
) which was used on location in the North Sea, and two more scale models for underwater shooting (in the studio resp. private pool of the Special FX guy. They also shot the depth charge explosions in this private pool, much to the dismay of the Special FX guy's wife).
As a German, I recommend the full-length mini series of
Das Boot, with the subtitles made for British TV. There's a set of subtitles used on US TV that's much more toned down in profanity and political commentary. There's also the Director's Cut, which is slightly shorter, but also OK. Better quality and better sound than the series, but, as said, running a bit shorter (as far as I can tell from the commentaries, the mini series WAS intended to be the DC, re-mastered and with re-done sound, but some of the original negative was irretrievably damaged, which is the only reason it's not included in the DC).
There were two dubs, one for the original movie and series, the other for the DC. The first dub featured only a few of the German actors; the rest are being overdubbed by British actors. The dub is very much toned down in profanity, but the subtitles are pretty close. The second dub for the DC is much better, and features most of the original German actors (who were all fluent in English). It's acceptably close to the original, but the subtitles are still better, though I think that the subtitles made for the original movie and series are still better and more correct than the ones for the DC.