See my response in your OTHER post. But if you are really concerned on it getting sold out, go ahead and book now on line. If you don't go to Jamacia for storm reasons, you will get a refund, so as long as you are sure you want to go AND you are willing to pay the ship's price, then by all means, book online. But before you do, google on "dunn's river falls tours" and see if you find a private company and find out what they charge.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007