Rowan and the Keeper of the Crystal — Emily Rodda

For generations, Rowan's family has chosen every new Keeper of the Crystal in the seaside village of Maris. This time, Rowan accompanies his mother to perform this duty. Then, when she is poisoned and near death, he must become the Chooser.

But first he must solve an ancient riddle and outsmart a deadly sea serpent in a quest for the poison's antidote. Can he make wise choices to save his mother, himself...and even braver ones to save all of Rin from a bigger threat looming just beyond the horizon?