To add to what Dave and Neil said, for Montego Bay, there is shoppig within the enclosed pier area that the locals are not allowed into. However, as Neil says, there are better shopping opportunities on most cruises that include Jamaica. Since the shops ARE kept from the locals, the prices are higher since they know you are only shopping there to avoid going outside. I wouldn't waste my time.
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