If you want to use the waterpark, the best option is to rent a day room and you get to use the facilities.
In the past you could rent a day room at Comfort Suites and get to use the facilities. I am not sure if they are stilling doing that, I would call and ask them. It is quite a nice option, as they have a change room in Atlantis and one can walk all over the grounds. Plus if you are like me it is fun to lie in the lazy river or sit in the nearby hot tubs.
It was a great way to see the resort and one of these days we plan to go there for a vacation.