Tides of Mana by Matt Larkin
She controls the seas. Her sister controls the flames. Together, they rule as god-queens over their Polynesian island paradise. No mortal army can stand against their power. Power, matched only by their pride. But what happens when civil war erupts between these goddesses? Namaka turns the fury of the sea on her sister, wreaking untold devastation on the land and under the sea, earning the ire of the mer kingdoms. Their answer: turn Namaka into one of them. Possessed by a mermaid spirit, she is drawn into battles in their alien world. How will she survive a war not only with her sister, but with a rival mer nation?
Book Length: Long Novel - 320-650 Pages