One caveat: Many cruise companies prefer to sell through travel agents, so it's often best to book through an agent who specializes in cruises.
American Canadian Caribbean Line
Grande Caribe, Grande Mariner, Niagara Prince
American Cruise Lines
American Eagle, American Glory
American Safari Cruises
Safari Escape, Safari Quest, Safari Spirit
American West Steamboat Co.
Empress of the North, Queen of the West
Carnival Cruise Lines