Black Dog of the Sea by Natasha Ruhwald

Raised by peaceful descendants of dryads, Corvin always feared a darker nature might lurk inside him.

Corvin is forced to watch helplessly as his family’s home is scorched by a pyromancer’s demonic fire. To spare them further harm, he must agree to guide a voyage across waters infested by mer pirates and haunted by an ancient evil, using powers he barely knew he had.

But others on the voyage are not happy to have him aboard, including Lady Hexton, who was forced to undertake the voyage by her father. And Commander Hardeton, a blood-thirsty man who is far more than he seems.

When the crew’s distrust of him finally boils over into a violent mutiny, Corvin is forced to take a life to escape. He came on the voyage to protect the adopted family he left behind, but now he seems to have more in common with the monsters around him.