Fenn Halflin and the Fearzero by Francesca Armour-Chelu

In a world that has been ravaged by flooding, Fenn Halflin learns the truth about his past - a secret that forces him to flee the safety of his home as he is chased across the vast sea by the ruthless Terra Firma and their cold-blooded leader, Chilstone. Finding shelter on a decaying island, Fenn is taken in by child survivors in hiding, and he begins a deadly game of cat-and-mouse with the Terra Firma, who will stop at nothing to find him. But as Chilstone's dreaded Fearzero ship appears on the horizon, and the water levels continue to rise, it seems that Fenn's hopes of escape will be washed away for ever.