"Das Boot" is one of the few movies, that should be watched in german.
??
Do you mean, as opposed to having it dubbed into English?
If so, I heartily disagree. I find it very distracting to watch foreign language movies dubbed into English. I much prefer to watch and hear them in their native languages and read the subtitles.
Back in the 1980s Mum and I used to enjoy watching the (*ahem*, how times change) West German cop show "Derrick". One of the standing jokes we shared was our inability to watch the show with the sound turned down - we both much preferred hearing the actors speak, even if we understood virtually nothing of what they said.
By contrast, I seem to remember that the mini-series "Mussolini and I" featured a large number of English-speaking actors, whose voices were then dubbed into Italian, and the show screened in Australia with subtitles. Once again, with lips not matching words, I found the experience distracting - I kept trying to see whether what the actors was saying (in English) matched the English subtitles.