Because there wasn't a lot of detailed information on Atlantis when I searched before my cruise, I'm going to talk about the things I would have liked to know.
We initially booked the "Discover Atlantis" excursion through Royal Caribbean. Our tickets were waiting for us in our stateroom when we boarded the ship. On the tickets there was a printed message stating that the aquarium was under construction. I went to ask what that meant exactly at the shore excursion desk and was told that because of the construction there was no access to the aquarium. Since that is more than half of the tour, I asked for, and received a refund. While in port at Nassau, we did some shopping and then took the $4 taxi ride to Atlantis. Our driver was a wonderful, grandfatherly type man who gave us a "tour" during the drive. We tipped him well because of his friendliness. We didn't do any of the water attractions but the pool areas that I saw were beautiful and inviting. There are some absolutely beautiful works by Dale Chihuly in the lobby and on the ceiling of the casino! The entire resort is gorgeous!
We went downstairs to where the entrance to the aquarium is, and there is a desk where they sell the day passes for $49 (which is $10 less than what the ship charges for the excursion). There was no construction and we were told that there was some "updating" but that had been finished for a month. We bought the day pass and within 10 minutes there was a tour leaving. With myself and my husband, there were 5 people so it was essentially a private tour with a well informed, lovely young lady. It took about an hour and 20 minutes to see the aquarium and dig areas, as well as the area around the outside of the hotel and casino. After our tour we went to the casino to meet our friends. I was very disappointed that there wasn't a poker room (after watching on TV all the big tournaments they hold.) They are building a poker room but I didn't find out how soon it would be open. We played the slots for awhile because there were some that I had never seen, but only stayed for about a half hour.
To get back to the ship, we waited for a taxi in front of the main hotel area. The drivers there are not nearly so nice or accommodating. There were 4 of us but the driver wanted to get more people into his van so we had to wait about 15 minutes for more people who wanted to go to the pier. It was still $4/person but we didn't tip this driver at all.
I would definitely recommend a visit to Atlantis - even if it's just to walk around and see all the beauty.