Against the Tide — Tui T. Sutherland

The sun is shining in the Hundred Isles, and yet the path forward seems crowded with shadows. Conor, Abeke, Meilin, and Rollan have traveled across the world, seeking a set of powerful talismans in order to keep them from enemy hands. Throughout their journey the young heroes have been hounded by pursuers, who always seem to know just where to find them. Now they know why.

One of them is a traitor.

As they steer the crystal blue waters of this tropical paradise, the team can't help but suspect each other. There's a spy in their midst, and before this mission is over, a deadly trap will close around them.