Submarine

Submarines have so far had no place in the "canon", but according to the Secret Blueprints, England makes extensive use of them.

The Works
A number of cards in The Works are for submarines, equal in fact to the number of airships. Furthermore, scoring some of each kind is a shortcut to victory. Although The Works is not styled after military action, it gives one the feeling that an eventual conflict between the Air (The Baron's Peace) and the Sea (England) for World Domination may be  in the cards.

Possibly Relevant Outside Information
Captain Nemo's is probably too obvious to mention, but perhaps not to make a comment to that effect.