Deeplight by Frances Hardinge
The gods of the Myriad were as real as the coastlines and currents, and as merciless as the winds and whirlpools. Then one day they rose up and tore each other apart, killing many hundreds of islanders and changing the Myriad forever.
On the jumbled streets of the Island of Lady’s Crave live Hark and his best friend Jelt. They are scavengers: living off their wits, diving for relics of the gods, desperate for anything they can sell. But now there is something stirring beneath the waves, calling to someone brave enough to retrieve it. Something valuable. Something dangerous.
Nothing is quite as it seems, and when the waves try to claim Jelt, Hark will do anything to save him. Even if it means compromising not just who Jelt is, but what he is . . .