US Navy
Naval Historical Center: The h.l. Hunley in Historical Context
An extensive history of the Confederate submarine, H.L.Hunley, the first submarine to sink a warship. Also information about two earlier prototypes.
US Navy
Naval Historical Center: H. L. Hunley, Confederate Submarine
This site provides a short article on the history of the first submarine to engage and sink a warship. Many links to information about its history, preservation and excavation. Includes original US and Confederate Navy documents.
Other
On Deck: Submarines in the Civil War
The Hunley wasn't the only submarine used in the Civil War. Read this comprehensive look at the research into underwater warfare and the results of that research.