If you love waterparks, you really need to make Wisconsin Dells one of your vacation destinations. I can't imagine having more choice, both indoors and outdoors than what is offered there.
Of course it is not exactly in the south - except for Canadians!
Another idea is if you feel like heading further north to SE VA where I live. Busch Gardens is in Williamsburg - great trip in and of it self - and they have a very nice water park called Water Country USA.
We went to Water Country USA a few years ago - had one of the combo passes with Williamsburg and Busch Gardens. We got the tickets primarily for Williamsburg and Busch Gardens and really weren't planning on Water Country, but we decided to try it on our last day. We had a great time! We ended up wishing we had spent more time there. At that time we had a nearly-teen, a 7 year-old and a three year-old and there was something for all of them.
It might be a trip for you, but I like the Waterpark in Holiday World in Santa Claus, IN. I've been to every waterpark at the Wisconsin Dells and this one is my favorite. Best of all, free drinks and sun screen.
In Queensland, Australia, www.dreamworld.com.au is a theme park with waterpark next door and across the road and down from that one is http://movieworld.myfun.com.au which is a theme park with waterpark next door, also affiliated with Sea World on the Gold Coast (nowhere as good as your one's though)
My favorite Waterpark
I've been to every waterpark at the Wisconsin Dells and this one is my favorite. Best of all, free drinks and sun screen. 
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