What was the name of the world's first submersible vessel used in combat?

Turtle was the world's first submersible vessel with a documented record of use in combat. It was built in 1775 by American David Bushnell as a means of attaching explosive charges to ships in a harbor, for use against Royal Navy vessels occupying North American harbors during the American Revolutionary War.

On September 6, 1776, Turtle, manned by Sergeant Ezra Lee of the Continental Army, was used to attack the British 64-gun ship of the line HMS Eagle which was moored in New York Harbor. The plan failed. The darkness, the speed of the currents, and the added complexities all combined to thwart Lee's plan. Once surfaced, Lee lit the fuse on the explosive and tried multiple times to stab the device into the underside of the ship. Unfortunately, after several attempts Lee was not able to pierce Eagle's hull and abandoned the operation as the timer on the explosive was set to go off and he feared getting caught at dawn.

On October 5, Sergeant Lee again went out in an attempt to sink a frigate anchored off Manhattan. Reportedly the ship's watch spotted him and he abandoned the mission. The submarine was sunk days later by the British aboard a tender vessel near Fort Lee, New Jersey. Bushnell reported salvaging Turtle, but its final fate is unknown. General Washington called the attempt "an effort of genius", but "a combination of too many things was requisite for such an attempt to succeed." Despite Turtle's failure, Bushnell was hailed a Man of great Mechanical Powers.