The Highwayman’s Apprentice and the Reluctant Merman by Joanne C. Stokes

Jude moves to a seaside fishing village from Surrey with his family after his grandfather suddenly dies. His grandfather leaves him a pocket watch which causes events to unfold and change his life forever. You see, Jude has a secret family history and, unknown to him, it would begin a strange and breath-taking adventure.   A merman, Solorelle, is sent by the Mer King Abalone, who, together with the help of a highwayman, protect him from the evil master who wants his power.