The Call of the Tide. Sea Mage Cycle

Only he can unlock the power-if they don't overpower him first. After dropping out of the King's Fleet, Samuel didn't think he would amount to much. Though he possesses the gift of magic, he will lose it if he ever cuts his hair, giving him a disheveled appearance that makes him a social outcast. But when an ancient and secretive cult comes after him, he learns that his particular kind of magic sets him apart.
An ancient lost artifact known as the tidecaller's amulet is the key to unlocking a great evil, and Samuel is one of only a few people who can use it-or destroy it. As the cult forces him to flee for his own safety, he receives a royal commission to find the tidecaller's amulet and destroy it. With the help of a flamboyant privateer captain, he sets out to find the ancient artifact before it-or he-falls into the wrong hands.