It used to be that Cozumel was the best for silver, but they have gotten the capitalistic bug and while you cna still find a good deal, the deals are not as good as they used to be.
You can find good deals, but you need to be very careful in all the places you listed while buying jewelry. Unless you can tell the difference between plated metals, glass stones and imitation pearls and their real counterparts, you are taking your chances buying in these ports.
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, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013