Written in Water by Elizabeth Schechter

Aven’s world is frustratingly simple. He lives far out at sea for reasons that his parents will never discuss with him, telling him only that life on land isn’t safe. Then the discovery of a storm-damaged ship with a single survivor reveals the truth. He is the Waterborn, one of the five chosen by the Mother Goddess to save the world. And the world is far more complicated than he ever dreamed, and far more terrifying. Forced to leave behind the only life he’s ever known, Aven must learn quickly how to survive on land, and how to protect his newfound loves from the unknown menace hunting them. Can Aven fulfill his destiny and save the world? Or will this quest cost him everything?