Depending on when you are going, you may be able to get a discount (upsell?) for French Quarter for under $120. It's not really a typical waterfall and the slide is not exactly challenging, but if you wanted onsite, it's a thought.
Also, Sheraton Vistana Resort (the original one, near I-4, not the Villages) has one pool with a short, zippy slide and a couple others with waterfalls.
Marriott Orlando World Center has been coming up again with Priceline recently, if you are into a bit of risk-taking. Learn the ropes (& restrictions) before you bid, though.