Finally got my hands on this and got it to run.
Any abandonware stuff I tried didn't work, so I ordered a CD via Amazon, unfortunately I did not check that it is the German version, and now I have to endure German texts in an American submarine. Blasphemy.
However, I am SO hooked by this gem. I read over and over that this can be considered one of the most authentic titles, due to the fact that the lead dev was a 688 skipper in real life - what more can you ask for?
Although very old, it plays "comfortable" (many old games lack many control-related standards we take for granted today), runs well so far and despite some sound-crackling here and now especially in video sequences, it's all good.
Technically it offers some really interesting features, like having to catch comms windows with the VLF antenna to receive satellite transmitted messages etc, and updating the AO-map with help of recon satellites also just gives you the feeling to be part of something bigger.
The crew gives so great feedback, unlike in DW for example, where it, without spamming you, let's you know if it loses a contact on sonar, if it regains it, if it is breaking up, etc. etc.
And I just played the first 2 training missions, I probably haven't seen half of it.
Fantastic!
To those who haven't played it yet, but are generally interested in modern submarines - play it. You won't regret it.