The Confederacy might have had a third submarine operating in Hampton Roads, until a crew from the James River Blockading Squadron noticed the float supporting its snorkel and made fast a rope to the air supply line. That submarine has not yet been recovered. The Navy might also have had a submarine, although its first operable submersible was not ready until 1866.
The book relates an instructive story about changing strategic interests. In the 1850s, the European powers adopted a convention banning privateering. The United States, with a relatively small navy, opted out. Come the Rebellion, the United States sought membership in the convention, which would have obligated the European powers to capture or sink the numerous Confederate commerce raiders operating as privateers. (The submersibles were also privateers, but presumably not intended for an early Battle of the Atlantic.)
It's less good with its references to Jules Verne. Verne has sympathies for various underdogs, including Rebel blockade runners, and he might have learned of some of the Rebel efforts to build an electrically powered submersible. (I am not making this up, there are several references to electromagnetic engines in the book.) It would not be too far a stretch for Captain Nemo to have put his great fortune to adaptation of Ruhmkorff's apparatus shortly after the Rebellion. But only somebody not familiar with Verne's work could pen this.
Verne, after all, presents his novel's protagonist, Captain Nemo, as a bona fide misanthrope. Cynically and cowardly, inside his submarine boat the Nautilus, he revels in sneaking up on his prey, conventional surface ships -- and for no good reason, sending them to Davy Jones's locker.The quote is from page 48 of Hunley. The ships Nautilus rams in Twenty Thousand Leagues are, apart from the U.S. Navy ship that fires on it, British. Verne is French. Captain Nemo's ancestry is revealed in Mysterious Island. Nautilus is an allegory for Hunley, including the resentment of the gentry against an unsympathetic occupier.
(Cross-posted to Cold Spring Shops.)