Compulsion

Nada is seventeen and hungry for adventure when handsome, blue eyed stranger Frano Tollin arrives on the shores of the Adriatic island paradise Nada calls home. After a short courtship he whisks her away to a glittering underwater city, promising Nada a lifetime of adventure, excitement and most important of all, love.
But when Frano Tollin's hunger for power takes a dark, obsessive turn, Nada seeks solace in the arms of the one boy Frano Tollin forbids her to know, his mute younger brother, Lukas.

Compulsion is a prequel to the Submerged Sun Series but may be enjoyed as a standalone title.