If you like snorkeling then Champagne reef is a must. It is NOT hard to access. They built the walkway out to it so you don't have to walk over the rocks. (its a rock beach and not a sand beach)
We ended up negotiating private tours on each island. If you can find a group of people that want to do the same thing, you can get better deals. But on Dominica we just told them we would like to see the falls and Champagne reef and we got a tour that did those and a bit of the rain forest (which you had to drive through to get to the falls) and through the city to the reef for I think $30 each??? The reef had a $2 entry fee (you can rent snorkel gear there if you don't want to bring your own) and the falls had an entry fee. But the same tour minus the reef was like $90 a head from the ship..
The same goes for the other ports on the trip. We got tours on each island but we did book the Pirate Adventure and the Turtle & Shipwreck cove snorkel through the ship because those get booked up quickly. I recommend the Sea Turtle tour... (if you are into wildlife) it was the most calming thing to have a sea turtle swimming up to you ... (you needed to be facing AWAY from the crowd for this affect though)
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