The Wizard's Promise — Cassandra Rose Clarke

Hanna has spent her life hearing about the adventures of her namesake Ananna, the lady pirate, and assassin Naji. She dreams of the same adventures, but little does she know she is about to tumble into one of her own. Hanna is apprenticed to a taciturn fisherman called Kolur, and, during a day of storms and darkness, are swept wildly off course.
In this strange new land, Kolur hires a stranger to join the crew and, rather than heading home, sets a course for the dangerous island of Jadanvar. As Hanna meets a secretive merboy, and learns that Kolur has a deadly past, she soon realises that wishing for adventures is a dangerous game - because those wishes might come true.