Fevre Dream by George R. R. Martin

I would have liked to mark this as a five star but I found it stopped and started too much for me. Just as the tension was building, Marsh (the main character) was driven off whichever boat he had been on and had to hunt the Fevre Dream down again. It was a very good story, however, and I can almost see Joshua's potion being the early stages of Harris's True Blood.