It's funny that you should say that, as most of the voices in
Das Boot are dubbed anyway, due to the noise emitted by the special camera they used in the interior sets.
Still, dubbing German voices to German mouthing looks a lot better. I remember one time, I put the disc in, and started were I had left off. I enjoyed several seconds of "It's a long way to Tipperary", when Frenssen said "Here's to the end of the war lads!"
In a British accent.