The Mermaid by Shane Scollins

Some mistakes can shatter your world.
Jake decides he’s made his last mistake. His recklessness killed the two people he loved the most, and he can’t carry the pain of it any longer.
But when he puts the gun to his head, he doesn’t get oblivion. Instead, the voice of myth calls to Jake from the water, pulling him out of his desperation and into a mystery deeper than the sea that holds it.
Jake’s life was saved by a beautiful creature of the deep. Now, it’s his turn to save hers.