Its just that the wiki for the waterway=waterfall tag says it should be a part of a river or stream. 

Its also not really a waterfall but more of a spout of water to get people wet for a water ride. I've been on that ride a few times so I know what it looks like but heres a random youtube I found to show you in case you haven't. https://www.youtube.com/watch?v=aeWG7KJEwrs

The waterfalls are at 0:31 to 0:50 and 1:40 to 1:46.