The Ingo Chronicles — Helen Dunmore

In this companion novel to the Ingo series, myth and reality collide when Morveren, a young girl from a Cornish island, discovers a Mer boy, Malin, half-buried in the sand dunes. New conflicts erupt between the worlds of sea and air, and storm clouds of danger gather as Morveren and her twin sister, Jenna, struggle to protect Malin. An enthralling battle of loyalties begins when Morveren and Jenna learn that even your closest friends can betray you, and that a tragic reality lies beneath their island's legends.