Yeah. Remember what? What does Elsa know about Evie that even Evie herself doesn't??
My improbable guess is that Elsa also got isekaied and they were dating or knew each other on earth.
No no no.
It's nothing complicated. Elsa is convinced that Evie loves her. Or loved her. Everything Evie did to Elsa previously (before the "bad ending" that got her exiled) looks like conclusive proof in Elsa's eyes. Things like watch her all the time, pay attention to her moods and needs, protect her from harm, get into fights for her, take every excuse to touch her... even her bullying looked like the pranks a child plays to get closer to the girl she likes!
Like: pour water on her to have an excuse to dry her hair with a towel.
Or: hide her textbook to force her to share Evie's own, which leads to lots of casual touching.
And of course it was Evie who ran to the roof, rescued Elsa who was about to get killed and defeated the darkness spirit who had possessed her.
Because of this, Elsa was persuaded that Evie sincerely loved her. And she fell hard for Evie. And she ran after Evie when Evie got exiled.
But Evie wasn't happy when they met again, and since then she has been sending Elsa a ton of mixed signals. One day she acts besotted and possessive around Elsa, the next one she tells her to go back to the capital and find herself a boyfriend. One day she looks betrayed, hurt and desperate because some guy seems to be wooing Elsa, the next one she invites Garth to be Elsa's suitor.
It's no wonder that Elsa thinks she may need a potion to bring back Evie's "forgotten feelings" from the time they lived in the capital.
I think it could be related to the incident at the beach
So, it doesn't loke like a dangerous fluid by itself (after several hours mostly any kind of poison or chemical diluted must have created detectable vapors) but the inside is dangerous (objects under the surface) and drowing on purpose it's still a probability. From Aoya being that persistent at midnight (the sister reduced the red flag made him look just too stupid instead) I was at mode -seriously bring a knife with you at least- but at the point of everybody reacting late just made me think if I'm the paranoid or everybody looks stupid against -living in a country who 2/3 times its law of the jungle- folk