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.
I wonder how much of today’s technology started off as a fanciful idea for a story.