Sword of fire and sea - Erin Hoffman

Captain Vidarian Rulorat's great-grandfather gave up an imperial commission to commit social catastrophe by marrying a fire priestess. For love, he unwittingly doomed his family to generations of a rare genetic disease that follows families who cross elemental boundaries. Now Vidarian, the last surviving member of the Rulorat family, struggles to uphold his family legacy, and finds himself chained to a task as a result of the bride price his great-grandfather paid: The priestess Endera has called upon Vidarian to fulfill his family's obligation by transporting a young fire priestess named Ariadel to a water temple far to the south, through dangerous pirate-controlled territory. Vidarian finds himself at the intersection not only of the world's most volatile elements, but of the ancient and alien powers that lurk between them...