Book about a girl and boy who go play on the outskirts, near a farm, the boy climbs up high into a tree and the branch breaks and he falls into a very deep waterhole.
The girl tries to reach for him but she sees these mermaids pulling him deep down and she is unable to rescue him. She is then put into a mental hospital cause no one will believe her, she then gets questioned and describes the what happened throughout the book but she is traumatized.
I think this may be 'The Leap' by Jonathan Stroud. A girl called Charlie sees her charismatic and daring friend Max drown in a deep pool, although she tries and fails to save him by diving in. Recovering in hospital, she starts to feel Max is trying to summon her back to him. The rest of the book deals with whether Charlie is suffering from some ongoing guilt/physical trauma causing hallucinations, or whether she and Max accidentally breached a barrier into a supernatural dimension which has taken him, and wants to take her.
There's an article here: http://en.wikipedia.org/wiki/The_Leap
