Atlantia by Ally Condie

For as long as she can remember, Rio has dreamed of the sand and sky Above—of life beyond her underwater city of Atlantia. But in a single moment, all Rio's hopes for the future are shattered when her twin sister, Bay, makes an unexpected choice, stranding Rio Below. 

This book by the author of the popular Matched series is fine, but a little wanting.  The world-development of Atlantis could use richer details, and the characters need more back-story to explain their behavior. The romance is a bit bland, too. But overall, the story zips along with enough twists to keep it interesting and surprising.