1863   For the second time, the Confederate submarine H L Hunley sinks during a practice dive in Charleston Harbor, this time drowning its inventor along with seven crew members.

Horace Lawson Hunley developed the submarine from a cylinder boiler. It was operated by a crew of eight--one person steered while the other seven turned a crank that drove the ship's propeller. The Hunley could dive, but it required calm seas for safe operations. It was tested successfully in Alabama's Mobile Bay in the summer of 1863, and Confederate commander General Pierre G.T. Beauregard recognized that the vessel might be useful to ram Union ships and break the blockade of Charleston Harbor. The Hunley was placed on a railcar and shipped to South Carolina.

The submarine experienced problems upon its arrival. During a test run, a crewmember became tangled in part of the craft's machinery and the craft dove with its hatch open; only two men survived the accident. The ship was raised and repaired, but it was difficult to find another crew that was willing to assume the risk of operating the submarine. Its inventor and namesake stepped forward to restore confidence in his creation. On October 15, he took the submarine into Charleston Harbor for another test. In front of a crowd of spectators, the Hunley slipped below the surface and did not reappear. Horace Hunley and his entire crew perished.

Surprisingly, another willing crew was assembled and the Hunley went back into the water. On February 17, 1864, the ship headed out of Charleston Harbor and approached the U.S.S. Housatanic. The Hunley stuck a torpedo into the Yankee ship and then backed away before the explosion. The Housatanic sank in shallow water, and the Hunley became the first submarine to sink a ship in battle. Unfortunately, its first successful mission was also its last--the Hunley sank before it returned to Charleston, taking yet another crew down with it. The vessel was raised on August 8, 2000, and will now reside in an exhibit at the Charleston History Museum.

I think the latest CWTI or America's Civil War magazine had an article about Singer, and Hunley and the guys working on the torpedos and submarines during the war.  It was more focused on the torpedos, but pretty neat article.

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GrumpyDave wrote : It was tested successfully in Alabama's Mobile Bay in the summer of 1863,...

Yes, it was successfully tested in Mobile Bay.  However, that successful testing followed the loss of the first vessel in that same body of water.  There is some modern day interest in attempting to locate the original in the relatively shallow water of Mobile Bay.  There are persistent rumors of more than one “David” out in the bay.  Two ironclads were sunk in a Mobile Bay Delta river (I’m not certain which one) to impede Federal vessels going up river.  They are still there.  Again, the local talk, is one is under five feet of mud and the other under 20 feet.  There is more interest in the ironclads than submarines, Davids, or even the Tecumseh (sunk in the mouth of Mobile Bay during the Battle of Mobile Bay.)  The reason is the location of the ironclads are known.  Hurricanes Ivan and Katrina rearranged the bottom of Mobile Bay.  The buoy marking the Tecumseh is still in place but Tecumseh is no longer under that buoy.  There was a recent search for her and she was not to be found.  Opinions differ as to whether she was pushed further into the bay or the receding storm surge pulled her out into the open Gulf of Mexico.