Orphans of the Tide by Struan Murray

The City was built on a sharp mountain that jutted improbably from the sea, and the sea kept trying to claim it back. That grey morning, once the tide had retreated, a whale was found on a rooftop.

When a mysterious boy washes in with the tide, the citizens believe he's the Enemy - the god who drowned the world - come again to cause untold chaos.

Only Ellie, a fearless young inventor living in a workshop crammed with curiosities, believes he's innocent.

But the Enemy can take possession of any human body and the ruthless Inquisition are determined to destroy it forever.