Drowntide

From the back cover: High upon a cliff of the dark rock, on the edge of the great sea... ... Queen Amelyor's city was in danger. She alone possessed the power to sound the horns -- beckoning the sea mammals to guide their fishing boats to safety. But now, the Queen's powers were waning... And so, Keiris, her only son, ventured forth into the sea to find his long-lost twin sister. Perhaps she possessed the gift and could save the city. From the haunting strains of an ancient song to the forsaken isles of mermaids and watery creatures, Keiris soon uncovered the mystery of his heritage -- and a strange new destiny that called to him from the wondrous depths of the great sea itself...