Tempest series by Tracy Deebs

 Tempest Rising (Tempest #1)

Tempest Maguire wants nothing more than to surf the killer waves near her California home; continue her steady relationship with her boyfriend, Mark; and take care of her brothers and surfer dad. But Tempest is half mermaid, and as her seventeenth birthday approaches, she will have to decide whether to remain on land or give herself to the ocean like her mother. The pull of the water becomes as insistent as her attraction to Kai, a gorgeous surfer whose uncanny abilities hint at an otherworldly identity as well. And when Tempest does finally give in to the water's temptation and enters a fantastical underwater world, she finds that a larger destiny awaits her-and that the entire ocean's future hangs in the balance.

Published May 10th 2011 by Walker Books for Young Readers

 Tempest Unleashed (#2)

[...] A deadly battle begins raging at the hands of Tempest’s old nemesis, the sea witch Tiamat. As the dangerous war erupts, Tempest’s two loves—Kona and Mark, sea and land—will collide for the first time, both to protect her and to force her to choose.

Expected publication: June 5th 2012 by Walker Childrens