Hereafter by Tara Hudson

"Drifting in the dark waters of a mysterious river, the only thingAmelia knows for sure is that she's dead. With no memories of her past life, she's trapped alone in a nightmarish existence. But everything changes when she tries to rescue a boy from drowning in the river. Because even though she can't do anything to help, Amelia somehow wills him to survive. And when he wakes up, Joshua can see her."