Ganymede

From the back cover of the Tor UK first edition: "Josephine Early, New Orleans brothel owner and Union spy, has a mission. One that might just end the Civil War. She must deliver Ganymede, an astonishing prototype submarine to the North. But the giant war machine is at the bottom of a lake, no one has successfully piloted it and she must sneak its huge bulk past enemy forces. Luckily, she knows the right man for the job. She hsn't seen former lover and air-pirate Andan Cly for years, but has a sweet job to tempt him back. He agrees to help, but his primary mission is to retrieve supplies for blighted Seattle, where noxious gas has forced residents underground and undead rotters menace the city. Although legal, the Seattle run make Cly uncomfortable. He no longer flies sap, disapproving of the drug's ugly side-effects, and the job is funded with sap money. Josephine's jaunt could be a great distraction. But will they end up in the history book, or at the bottom of the ocean?"