Suncity in South Africa had opened a water slider like that. You're made to stand straight in a pod, the floor is dropped from under you and the speed propels you around the roller coaster part
I had to do a double take on your comment because here in the U.S. some states have "Sun City" communities (by Del Webb and maybe others) that are for 55 years and older (like old folks communities). I just imagined them having an intense water slide.
https://memegenerator.net/img/instances/500x/66290778/sploosh.jpg