All the Stars and Teeth (The All the Stars and Teeth Duology) (All the Stars and Teeth Duology, 1)

Set in a kingdom where danger lurks beneath the sea, mermaids seek vengeance with song, and magic is a choice
She will reign.
As princess of the island kingdom Visidia, Amora Montara has spent her entire life training to be High Animancer--the master of souls. The rest of the realm can choose their magic, but for Amora, it's never been a choice. To secure her place as heir to the throne, she must prove her mastery of the monarchy's dangerous soul magic.
When her demonstration goes awry, Amora is forced to flee. She strikes a deal with Bastian, a mysterious pirate: he will help her prove she's fit to rule, if she will help him reclaim his stolen magic.
But sailing the kingdom holds more wonder--and more peril--than Amora anticipated. A destructive new magic is on the rise, and if Amora is to conquer it, she will need to face legendary monsters, cross paths with vengeful mermaids, and deal with a stow-away she never expected--or risk the fate of Visidia and lose the crown forever.
I am the right choice. The only choice. And I will protect my kingdom.