The Devourer

Adrasteia Dantes always knew she'd be a pirate captain. She just didn't know she'd be seventeen years old when it happened.
Adra takes the helm of the Worldeater after her half-brother Cameron attacks their father and steals a priceless treasure map. Now, the young captain has only one desire: revenge. She will restore her father's honor by killing her brother, reclaiming the map he stole, and finding the legendary treasure at the grave of the first pirate king.
But when Adra discovers her brother's ship destroyed and his crew dead, spare a sole survivor, she learns there's something lurking in the sea that's killing pirates, taking entire ships down with ease. Adra takes the girl prisoner in hopes of tracking down Cameron, but no one will survive to see the treasure if they don't find and stop the Devourer first.
If Adra wants to save her ship, she'll have to strike a deal to help the Devourer reclaim something that was taken from her long ago. But it seems that their individual quests for vengeance might be leading them to the same end . . . .