Cynthia Leitich Smith
Check out the book trailer for The Vicious Deep by Zoraida Cordova (Sourcebooks, 2012). From the promotional copy:
When an unnatural riptide sweeps lifeguard Tristan Hart out to sea for three days and then dumps him back on the shore of Coney Island, it's the start of the Sea Court claiming its own.
Suddenly, Tristan's girlfriend dramas and swimming championship seem like distant worries as he discovers the truth: he's a Merman.