The Dark Tide Book by Alicia Jasinska (Author)

Every year on St. Walpurga's Eve, Caldella's Witch Queen lures a boy back to her palace. An innocent life to be sacrificed on the full moon to keep the island city from sinking.

Lina Kirk is convinced her brother is going to be taken this year. To save him, she enlists the help of Thomas Lin, the boy she secretly loves, and the only person to ever escape from the palace. But they draw the queen's attention, and Thomas is chosen as the sacrifice.