Laura Anne Gilman 
From Whence You Came 

Centuries before the Vineart War, massive sea-serpents roamed the seas, endangering the lives of all who sailed there. Only the greatest magic could keep them in check, and protect the coastlines from their depredations. And then…they disappeared.Legend credited Master Vineart Bradhai, and called him a hero.Legend…. is not always accurate. And often, the truth is a better story.Set in the world of the Nebula Award-nominated Vineart War trilogy