The Seventh Princess — Matt Larkin

In every generation a Princess is born to each island of Sawaiki, blessed with the power to control a single aspect of the natural world. Namaka's power is arguably the most valuable of them all -- in a world of archipelagos and endless ocean, she control the seas themselves. The mer people of the Worldsea despise her for this power over their domain, all the more so because she can barely control her own gifts. At war amongst themselves, they harness this uncontrolled power by possessing Namaka with a mermaid spirit -- turning her into one of them. As Namaka struggles to accept this new existence, she's faced with conflict above the waves and war below. And if she cannot control her powers, both her worlds face annihilation.