Not sure if they’re open now, but there’s Coco Cay in Orlando with it’s built-in water park. If you wanted some beach time too, the Holiday Inn timeshare at Port Canaveral has a pretty fun water area plus mini-golf and all sorts of other kids activities and it’s on the beach. Nether of those are very swanky resorts, but we’ve stayed at the port a few times and enjoyed it.
We’re more fond of the gulf beaches, but I really don’t know of any with a particularly great slide. There is a Margaritaville resort on Pensacola Beach which is fun if you’re an aviation buff—it’s the home of the Blue Angels and you can visit the air museum there.
And not at the beach, but the Great Wolf Lodges seem pretty fun and you’re not dependent on weather. Of course, when I think waterpark, I think outside, but throwing out the idea. Plus most outdoor waterparks (besides Disney) close after Labor Day, so think of that.
Hope that gives you some to ponder until someone with better info comes along!