Deeplight by Frances Hardinge
On the streets of the Island of Lady’s Crave live 14-year-old urchins Hark and his best friend Jelt. They are scavengers: diving for relics of the gods, desperate for anything they can sell. But there is something dangerous in the deep waters of the undersea, calling to someone brave enough to retrieve it. When the waves try to claim Jelt, Hark will do anything to save him.