Aquanauts is a Soviet Sci-Fi film. I remember seeing it when I was about 13 or so. It didn't have many special effects but it was very interesting and intriguing.
The plot (as far as I remember) goes like this: in the near future there is technology that allows people (aquanauts) to stay very deep underwater without the aqualungs or any other device. While doing their work on the deep underwater station those aquanauts discover strange mystery: it seems as if one of the aquanaut's girlfriend who recently died in a car crash had turned into a big manta fish. That manta writes words on the station's port-holes and tries to draw attention many other ways. So, aquanauts try to solve that mystery.
The movie uses classic approach of the Sci-Fi: use science fiction (by the way, in Russian sci-fi sounds like English word "fantastic") to show/explore our today's realities. Aquanauts deals mostly with human relationships though. There is no communist ideology or global philosophical thoughts there. It has some romance and drama, and some thoughts about humanity and our place in the nature. Overall, pretty good movie, all I can say.