I'll change the highway from "water_path" to "path" for now, those ways were meant to indicate traffic flows within a pool area, both after exiting a water slide and for approaching stairs. Is there a tag that I can use to indicate that the path is used for these purposes, or simply to indicate that they are paths within the water? I could add swimming=yes to these, as I had with the lazy river (which this tag is also not listed on the wiki for highway, should I change it?), although I don't know if I should be using key:swimming in the same way as key:foot. 
Also, I am going to mark most of the waterpark features as indoor=yes to fix them being rendered when only trying to render normal outdoor features (except for the slides, because they run outside the building, unless you think it would be better to mark these indoor=yes also, since they are only accessible from the inside. I also wanted to add a level tag and stairs, to better indicate the levels you must ascend to reach the top of the slide, although I don't know what level the water slides would be marked as, since they go from the top [about the 2nd or 3rd level or level=1 or 2] all the way to the bottom).
As for the untagged way, it is a bodyboarding machine which has water flow up a slope so that people can bodyboard on it. To the best of my knowledge it has no other purpose, and it is not deep enough to swim in anyway, so marking it as a swimming pool seems incorrect. I don't see tags for bodyboarding on the wiki, so maybe the surfing tag could be used there.