I was just reading sections of Thomas Pakenham's The Scramble for Africa
dealing with South Africa and Rhodesia, and stumbled onto the chapter about German Southwest Africa, now known as Namibia. The German colony in the South Atlantic would have to feature in the film somehow, as a destination or escape destination or something, because the timing is so right. I think the experimental deep-sea U-boot should have portholes or viewing windows made of neoprene or some experimental new hardened plastic material, and the twilight world below 1,000 feet should be lit up by external incandescent flood-lights from the submarine ship, so that the dancing patterns of the sea-life and luminosities can remind the captain of a winter scene in his native Prussia or wherever, and later he can become increasinly unsure, along with the rest of the crew, whether he isn't seeing human forms cavorting out there amid the luminous bits.
( Incidentally, while the initial Baltic UFO was hyped as being shaped like the Millennium Falcon, it actually has a definite U shape in the sonarogram. It could be the mother of all U-boots