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.
I think it was the tuna.
I also used all-electric propulsion by moving ferrofluids in hydraulic pistons to sweep the fins. No bang and clang of pumps and valves.
My sub used expanses of gill-like devices to extract oxygen from the ocean for combination with the fuel in the fuel cell. It was the best I could do without Nuclear.
I’m not waiting a hundred years for the future to arrive, I’m making it up now.