You can SAVE money by booking excursions on your own and not through the ship. Almost half!
But you need to be careful on times. Make sure you give yourself time to get back to the ship. Another thought would be to go to the Caribbean ports seciton of the forum and ask for recommendations on tour guide companies. If someone has god experience with guides (or bad for that matter) they are normally happy to share. If you were going somewhere else i could give you some names, but its been awhile since I have been to the Bahamas. Be sure to tell them which ports you are visiting.
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