You mean waterfall? The 'actually' waterfall will unlock after you finished Curse of lust event in good ending. But if you mean 'Nauxus take a bath at waterfall' scene. You can trigger this scene randomly (RNG) when you visit Nauxus after you finished his quest (not all quest, just a quest about talisman). When you visit him and event is trigger, there will be 'a eye' icon appear at top-left of your screen, you need to click it to see a CG.
This is where I am at right now, and is there any way to trigger this? Sorry about the picture quality 