I worked up a submarine for a potential novel.
It was a “run silent” civilian model, operating on fuel cells. It used an artificial whale design, with fins and flukes instead of propellers.
Not so fictional....
I was reading (I forget where) that some university is doing research with the Navy on a propulsion system for underwater drones that is modeled on the fin motion of some fish—can’t remember that either—that gets the most efficient propulsion from the movement of its’ fins.
I would love to be here 100 years from now to see what marvels the human mind will dream up. That’s IF humans are still here.
Just stay away from Japanese ships!