Most likely Germany, they had the U-boats (underwater boats)
The New Virginia Class attack submarine are the most advanced subs in the world and there deployed from the U.S.A.
There are over 200 submarines in the world currently. Of these, North Korea has the most with 78 and the United States is second with 72.
The most successful submarine of WW1 was the SM U-35 Type U 31 U-boat. It sunk 224 ships, not 223.
David Bushnell is credited with inventing the first submarine used in combat, known as the Turtle, during the American Revolutionary War. His creation was a significant advancement in naval warfare and laid the foundation for the development of modern submarines.
One successful tactic Germany used in World War One was submarine warfare. It lacked the navy Great Britain had. Building submarines was the fastest and least expensive way to have a military presence in the seas and oceans.
The Confederate States most successful submarine was the H L Hunley
Periscope
England had the most powerful surface fleet of ships, while Germany had the most powerful submarine fleet. England's surface fleet won out over German submarines.
Most submarines were made of steel.
Diesel-Electric submarines had been in use for years before WWI; there were many commissioned boats on both sides of the war before it even started, so there is no one particular submarine used in WWI. However, the one boat that arguably had the most impact at the beginning of WWI was the German submarine U-20, which was responsible for sinking the British passenger liner RMS Lusitania. This single submarine attack had repercussions throughout the war, causing policy shifts in Germany considering submarine warfare, and which affected submarine warfare in general forever.
The Confederacy led in the experimentation with submarines. The most successful was the HL Hunley. This submarine sunk a Union warship however the sinking ship damaged the submarine that also sank.