Fark linked up this interesting account of the discovery of what could turn out to be the inspiration for Captain Nemo's Nautilus:

She was built in 1864 by a visionary craftsman, Julius Kroehl, for the Union forces during the American Civil War. But the boat, called Explorer, was never used in the conflict and was subsequently taken to Panama where she was used to harvest pearls.

She was ideal for this purpose because of a unique lock-out system, identical to the one in the Nautilus from Verne’s book, published in 1870.

Of course, no direct evidence exists for a connection, but the parallels are pretty interesting. As with its near-contemporary Hunley, this sub's crew perished on-board, only instead of trying to sink ships they were harvesting pearls.