Wonders under the sea?
Defeating the submarine?
The most common suggestion to improve ship security against submarines during World War One was, without a doubt, a metal net mounted at some distance from the hull. The incoming missiles were supposed to deflagrate on the net, limiting the damage to the vessels.
This expedient was tried several times and in different forms, but the net always slowed the ship down without offering significant protection against torpedoes. The idea was suggested so many times that the Naval Consulting Board (the US government body charged with the coordination of national war inventions) officially asked the inventors to abandon it, focusing instead on the ships' speed and on warning mechanisms.
A futuristic and probably never realized version of the protection net is the "Torpedo-proof wall of steel", published on Popular Science Monthly and Popular Mechanics. At the sight of an approaching torpedo some cannons would have shot hundreds of rotating steel disks in the water. Their rotation was supposed to keep them in a vertical position, creating a wall between the torpedo and its target.
The metal nets and underwater mines were used successfully to protect stationary targets such as straits and ports. The inventors of the time proposed several improvements to these technologies. Among them, for example, a submerged buoy piloted by a single man that could rotate on itself and shoot torpedoes in every direction and a semi-submerged buoy armed with torpedoes and a machine gun on the surface. Its enthusiastic inventor calls it a "U-boat swatter" and imagines a chain of them spanning from Scotland to Scandinavia.
The "Gun-Buoy" is instead able to give shelter to four people, with beds, toilette, telephone and machine gun. When a submerged net is touched by a submarine a light blinks in the cabin, warning the crew that is time to spring into action. It is not clear what stops the submarine to shoot first, or the effect of marine life on the warning system.
As mentioned the submarine is a weapon mostly associated with the Central Powers. The imagination of the Allies was mostly focused on defense and sighting mechanisms, not on the evolution of the technology itself.
Some innovations were nonetheless proposed. While the first remotely controlled torpedoes were experimented upon, some inventors imagined that a pilot could steer the (submerged or semi-submerged) missile toward the target, leaving it just before it exploded. The pilot had to hope to destroy the enemy ship in one hit, because he was supposed to row away from the scene.
Italy was at the forefront of innovation in piloted torpedoes. An experimental model (semi-submerged) was used to sink an Austrian ship in the Pola port in 1918 and other submerged models were used during the second world war. Contrary to what was imagined during the Great War, none of them was used in open sea, had the pilot sitting inside the missile or was supposed to explode in its entirety against the target.
"Submarine midgets, and not dreadnoughts" wrote Popular Science Siftings in 1918 "may even yet prove to be the secret to ocean supremacy". Why not then create an even smaller one-man submarine, armed with just two torpedoes, that could be released from a ship? Some "pocket submarines" were used, with little success, during the second world war. Today many models are commercialized for civilian use.
Many solutions were proposed for the rescue of sunk submarines and their crews. One method involved a detachable turret where the sailor could take shelter in case of critical damage to the ship. Thanks to its oxygen tanks the turret, once unscrewed from the submarine, would have darted to the surface and to safety.