Tideborn

A dragon queen, a vengeful sea witch, and a mythical titan converge on the underwater city of Tiankawi in the sequel to the international bestselling epic fantasy Fathomfolk.
  
The people of Tiankawi have been irrevocably changed. Once divided into two factions – humans and the underwater ‘fathomfolk’ – now every citizen is able to breathe underwater. But this new commonality is not enough to heal the scars of years of oppression. Mira, a siren and activist, must uncover a deadly conspiracy while persuading her divided people to band together to rebuild their city after a tsunami. And dragon princess Nami must undertake a daring sea voyage to do the to find a way to convince a Titan not to destroy their city for its crimes.