I read a book in middle school a few years ago about two teenagers who take a boat trip. The girl is a bit of a daredevil, so she goes for a swim. The boy stays on the boat, and sees another boat that isn't paying attention, which hits his unsuspecting friend. She doesn't appear above water for a while and he gets worried. When she does appear, she's pretending he saw things and she's calm and seemingly isn't even hurt.
Later, she becomes weak again and finally tells him about the glowing green liquid she found in a cave that saved her life. Her cuts are back and so he takes her to the green water in the cave. They bottle it up so he can run some tests and so she can have it with her.
Then some important people get involved (I can't remember what tipped them off); they threaten him but he protects her. She is getting closer to death as the green liquid's effect fades. The bottled green liquid loses its glow, so it's no good. Eventually, they are found when he tries hiding her and then he performs a major escape from the hospital with her. He takes her to the cave and while she's happily restoring herself in the glowing green liquid, he traps her in there and leaves.
That's all I remember and it was extremely close to the end of the book by then.