Here’s a simple way to deal with enemy submarines — hang giant electromagnets over the sides of your ship:

The magnets being projected outwardly from the ship’s sides, a submarine within the fields of the magnets is attracted and drawn thereto until the glass caps are struck by the submarine and broken. The contacts will now be against the submarine which will close the circuit through the submarine, lighting the lamp and ringing the signal bell, thus notifying the crew of the ship of the capture of the submarine. The submarine will also be electrified, shocking the crew thereof and killing or rendering them temporarily helpless.

Inventor Louis Schramm offered the scheme in 1914. I don’t know whether anyone tried it out. Let’s hope not.