Riverkeep

When Fifteen-year-old Wulliam's father is possessed by a dark spirit, Wull hears that a cure for Pappa's illness lurks in the belly of a great sea-dwelling beast known as the mormorach, and so he embarks on an epic journey down the river that his family has so long protected - but never explored.
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.