Riverkeep - Stewart, Martin

Fifteen-year-old Wulliam is dreading taking up his family's mantle of Riverkeep, tending the river and fishing corpses from its treacherous waters. But then everything changes. One night his father is possessed by a dark spirit, and Wull hears that a cure lurks deep within the great sea-beast known as the mormorach. He realizes he must go on an epic journey downriver to find it - or lose Pappa forever.