It is that lake.
I think you might need to get 2 flag beforehand.
1. Left of the tower next to the lake there's a guy there, talk to him and he'll mention rumors of sound of water splashing at night.
2. Get enough affection with Relu (the purple hair) to have the event occur.
Btw, after that event the next time you talk to her, talk a few times until you get a small scene and she'll reveal herself to be female, some choices ensue (I think something like "are you gonna tell everyone?" (ANSWER NO) and then "Can I ask you not to tell anyone?" (YES YES YES YES) and from then on anytime you talk to her there'll be image just like all other girls)