Mel...I checked this out for my upcoming cruise on the EOS. We are in Nassau for 6 hrs and wanted to hop over to Atlantis and use their facilities. I called Atlantis and was told they USED to offer a day pass but no longer. You can see the facilities through the excursion on the ship...or walk part of the grounds on you own. But without being a guest at the hotel you cannot use the beaches or the pools and slides. They issue their guest wrist bands and I hear they can be pretty strict about them. some of the locals can be down right nasty, which has been my experience with Nassau in the past.
I also checked the Hotel next door and thye don't have a day rate but if you book an OVERNIGHT STAY you can check in, use Atlantis next door and then leave with your ship...but I found this too expensive for just a few hours of play time.
We ended up booking the Blackbeard Cay Beach excursion with the ship. 4 hrs on the beach...let's hope it doesn't rain! :grin