Interesting read. Some nitpicking as a born East-German: In my opinion, the last row of images shows pretty standard UW-orienteering equipment, which has not much do with Boettgers scooter-like device. In particular, it has no engine parts, only air bottle + navigation (compass and distance counting meter)
I'm quite sure, because I was involved in this kind of sportive activity during the eighties and nineties of the last century and have build this and similar equipment myself.
Not the left one - look at the three peoples physique. IIRC, the KSK participated in non-military competitions at the time. The image could show some of them.
The second (and also the first) one shows a device design not seen before the eighties, I think.
https://en.wikipedia.org/wiki/Underwater_orienteering
I'm quite sure, because I was involved in this kind of sportive activity during the eighties and nineties of the last century and have build this and similar equipment myself.