Elaine09- Lots of people on this site have used Captain Marvins in Grand Cayman. I don't have a good recommendation for Jamaica guides as they are not mentioned here much.
Be careful with other posters on this site (especially new comers) claiming to know a great tour guide for Jamaica as many to most times the posters turn out to be tours guides trying to drum up business.
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