Submarine anarchy mayhem challenge: Design any submarine for any purpose from 1905-1995. It doesn't even have to be military, post submersible megayachts and speculative deep sea explorers to your hearts content. Only requirement is that it must be able to fully submerge underwater and not kill the entire crew doing so, and be able to surface after the fact.
Hm, a change to this:
Design a cold-war fleet submarine, Hard Mode: In this world, nuclear power was never a thing. Diesel-electric, HTP, extremely large banks of batteries, gerbils, anything but nuclear can power this, as long as it has either huge range, good speed, or ideally both.