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