It was almost 5 years ago when we visited the aquarium and we did booked an excursion through the ship. I think it was around $40 per person but not sure. De&Rich are probably correct. Always better to go on your own and not through the ship.
As for Cabbage Beach, we were there this past October. We got a cab for $3 per person and went straight there. Bring your towel from the ship and you can sit on it or rent a chair. They also have jet ski rentals and bannana boat rides if you are interested. We had such a good time here that we are going back for Thanksgiving.
I would take a cab to Atlantis first, look around then walk to Cabbage Beach. You can catch a cab back to the ship from there. Hope this helps.