Long story short - I booked Blue Lagoon directly through a 3rd party and when I got to the pier was told I did NOT have a reservation, even though my credit card had been run and I had a confirmation number. I was told my confirmation number was bogus. The Blue Lagoon rep told me that on days the Dream or Fantasy is in port they sell ALL of their spots to Disney and there is no way to make a valid reservation other than going through
DCL. She said that Nassau 3rd party companies know this but continue to book reservations they know they don't have the right to make. This was back in June, 2012 so things may have changed since then.
Happy ending - the Blue Lagoon rep knew the loser that took my money, and called him up and blessed him out. She got him to refund my credit card (they even brought me the credit card refund slip when the next Blue Lagoon boat came over to the island) AND gave our entire family a free Dolphin Encounter for our troubles. (We had booked a Swim With The Dolphins, but that was entirely sold out for the day and there were not 4 openings.) The Blue Lagoon people couldn't have been nicer. We had a great day and I highly recommend them.