What was the first submarine to launch a torpedo at an enemy ship during war?

The Greek submarine Delfin became the first submarine to launch a torpedo at an enemy ship (albeit without success). In 1912, Delfin launched a torpedo against Mecidiye from a distance of 800 meters, but failed to sink it. It was noted at the time that the torpedo broke the surface and passed by the ship. This is the first recorded launch of a torpedo by a submarine in battle.

In its attempt to return to Tenedos, the Defin ran a ground on a shoal north of the island, and had to drop its lead ballast in order to escape. This meant that the ship was no longer able to submerge. Therefore, it sailed back to Piraeus. Ultimately, the Delfin, along the rest of the Greek fleet, were confiscated by the French in 1916.

Also, it should be noted that Delfin (dolphin) was a Greek submarine that was actually called a "submersible", and it served during the Balkan Wars and World War I. It was the second submarine to enter service in the Greek navy, after the Nordenfelt I, which served from 1886 until 1901.

They were a French built class of submarine; and, they were both built for the Royal Hellenic Navy in 1910. Both submarines were seized by France during the Greek National Schism and used during World War I; ultimately, they were returned to Greece.