When Fire Loves Water Part I

Part I: The Siren ... Everyone knows that Fire hates Water
Especially the Sireni of the Sea, and the Daeveni of the Smokelands, whose millennia-long conflict with each other has escalated to war.
Isla Wilson is a human girl who just wants to graduate high school so she can find a place where she actually belongs. Then she drowns. Given new life by the mermaids, she finds they want something in return: her. The Sireni believe she will lead them to victory against the Daeveni.
Taz Howell, an entrepreneur and college hockey player, is sacrificed to a volcano by his father. He survives. Kind of. Reborn in the Smokelands, he is now Daeveni. He will lead the fire-ghosts with tectonic powers against the greedy Sireni.
Behind the scenes, masterminding a plot to wash the world of humanity, is Taz's father. His plan—escalate the neverending battle between fire and water until nothing is left so he can rebuild civilization in his own image.
In their new bodies, Isla and Taz are mortal enemies. Except that a chance meeting as kids changed them both, forever. What might happen if Fire met Water again? Could they save humanity?
What would the world be like if Fire Loved Water?