We did the walking tour of Nassau (our own made up primarily from Frommer's) and the glass bottom boat that leaves from the same pier where the ship docks.
My then 13 yo LOVED it. It goes over a reef or two and a shipwreck and was cheap.
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04