Rowan and the Keeper of the Crystal. (Rowan of Rin series, #3.)

Rowan and his mother have been called to Maris to choose the new Keeper of the seaside village's precious Crystal, a dangerous task Rowan's family has been bound to for generations. But this time, something goes terribly wrong. Rowan's mother is poisoned and near death, and Rowan must find the antidote, outwit a deadly sea serpent, and pick a new Keeper -- before it's too late. Destiny leads Rowan of Rin to make choices which could save a powerful crystal, thereby ensuring the safety of his people and his people's shore-dwelling allies.