If you have more than one adult and one child, it will be cheaper to book a room at Atlantis or Comfort Suites to gain access to the waterpark and aquarium. I booked it through Disney because there is only DS and I. It was $260 for the two of us. It would have cost that to book a room. We are also guaranteed that if we don't dock in Nassau for some reason, we are not out our money.
We have never been, but it is DS's 9th birthday the day we are in Nassau, and he absolutely loves everything that Atlantis has to offer. We sail on 8/30/07 so I'll post back how we liked it.