The movie is very old. I watched a movie in my childhood, 25 years ago or more, and it was already an old one. I remember it as colored (under question).

It’s about an underwater expedition. May somehow be connected with Jacques-Yves Cousteau (under question). Divers go underwater and there they find that there is another world, which is upside down to the water (so the water divides the worlds, something like that – at least, I remember it like that). And there there are also humans.

It’s all I can remember. Thanks for help!