It sounds as if you're sailing on the same itinerary as we are. My buddy and I will be on the Royal Princess from Manaus to Ft. Lauderdale, departing Manaus April 14, 2008 (one week from today!)
I've been researching the dive possibilities in St. Lucia, St. Barts, and Tobago with only a little success. We have a two-tank boat dive booked in St. Lucia with Dive Fair Helen. They tell me that we can walk to the pick-up point from the cruise ship dock.
So far I have not been able to find anyone in Tobago who would take us out. One dive op said they were too far away. Another, World of Watersports, said that their schedule does not match our cruise ship schedule and that they could only accommodate us if they got more divers from our cruise. I haven't found anyone else yet. Are you on our cruise by any chance? Please PM me if you are.
In St. Barts, PLONGEE CARAIBES would be happy to take us, but the cost is 110 Euros per diver. As weak as the US dollar is, that's a lot of money. We haven't made a decision about that dive yet.
Have you been on your cruise yet? If you've already returned, I'd love to hear your experiences and how the diving worked out.