We took an extended family vacation to Atlantis a few years ago, and I will say we all had a ton of fun at the water park (over the course of the entire week we never even made it to the beach because there was so much else to do). The layout is huge, with multiple pool areas catering to different age groups (from kiddie/playground pools, to adult only quiet pools), and there's a wide range of water slides/attractions for all thrill levels. We'd spend countless hours on the wave river (think lazy river meets wave pool), there are multiple inter tube style water slides (with different levels of drops) and even a few steep drop body slides (one straight drop into complete darkness, and the other an exposed drop from a Myan style temple where you land in a clear tube which goes through the shark tank - you'll see pictures of this one if you go on their website).
However I will say that there was a noticeable difference in crowds/lines on the days when there were cruse ships in port vs. those were only resort guests were at the water park. So it may be worth trying to scope out how many other cruise lines will e in port that day, as the more crowded it is the less fun you'll have. It's a pretty expensive day pass, so I'd only suggest doing it if you're really into water slides (as the previous poster noted you don't have to pay to get into the casino), but cost aside it can be really fun. If you have any additional specific questions about Atlantis, just let me know!
. I so wish 
