How dependable was the submarine in World War 1?

Answer 1

Dependable enough to sink large vessels (RMS Lusitania, e.g.), merchant ships and warships.

Though ranges and armament were limited, the success of the German submarine fleet led to the Versailles Treaty having a condition that all German submarines were to be handed over to the Allies after the war. This was primarily so that the Allies could study German submarine technology in greater detail. The Germans were so far advanced in submarine technology that even their WWII technology legacy is incorporated into today's submarines (e.g., hull shape and design).

Only the change in policy toward unrestricted submarine warfare kept the Germans from achieving more significant victories against enemy shipping and warships. Had they continued their initial policy, WWI may have turned out very differently.