It's amazing how, in the world of submarines, the basics haven't changed. In World War I days, the most advanced submarines, and their predecessors, had many common attributes: a cylindrical shape, ballast tanks to allow the boat to dive, and periscopes. Fifty years later, I served in a Royal Navy A-boat.
Andrew boasted an atypical feature: she was the last RN boat with a deck gun.
The A-boat featured in my upcoming novel set during the Cold War will also carry a 4" gun.