Who invented the first submarine used in war?

Though David Bushnell's Turtle is the first submarine credited with use in combat during the Revolutionary War, it never made much of an impact as none of its attacks were successful.

The Confederate States' Army confiscated the privately built submarine H.L. Hunley (now being preserved in Charleston, S.C. after her discovery a few years ago) for use as a warship during the Civil War. The Hunley was privately designed and built by marine engineer Horace L. Hunley, James McClintock, and Baxter Wilson. Contrary to popular belief, the Hunley was never a commissioned vessel in the Confederate Navy; it was operated by the Confederate Army, and crewed by 8 Confederate Army soldiers and 1 Confederate Navy sailor.

Though the U.S. Navy also had a commissioned submarine in the Civil War (USS Alligator, 1862), the Hunley is considered to be the first submarine to be successfully used in war, and is also credited with the first sinking of a ship by a submarine during wartime. The Hunley successfully attacked and sunk the USS Housatonic, then anchored in Charleston Harbor. The Hunley sank on her return trip back to base following the attack.

Though it's long been believed she sank as a result of flooding as a result of the shockwave from her attack, her position when found, plus forensic examination of the wreck and crew remains has indicated that they in fact suffocated rather than drowned, as was widely believed at the time. This has led to the belief that she sank after her crew became unconscious and unable to power the boat. Regardless, her impact on naval warfare is undeniable, and would likely have changed naval tactics far sooner had she survived.