The only one I know of in the parks is at Epcot. After you pass the giant fountain and you're walking down the sidewalk towards World Showcase and you see the American Pavillion straight across the lake, it's on that path (I think). The last time I went to it was in 2004. It was me at 21 with a bunch of little kids. I was so hot.
I'll probably be utilizing it this time though for DD. I'll pack her swim diaper and let her go crazy. I don't think we'll feel the need to get her swim suit on her though, just one more thing to worry about. As long as she has plenty of sunscreen on, no real need for it imo. If your LO is in disposables, you could probably just leave their regular diaper on them and let them play, then just throw it out. We cloth diaper and re-use covers, so it would be a waste of a cover for us to let DD play in it.