Undertow book by Michael Buckley

Sixteen-year-old Lyric Walker’s life is forever changed when she witnesses the arrival of 30,000 Alpha, a five-nation race of ocean-dwelling warriors, on her beach in Coney Island. The world’s initial wonder and awe over the Alpha quickly turns ugly and paranoid and violent, and Lyric’s small town transforms into a military zone with humans on one side and Alpha on the other. When Lyric is recruited to help the crown prince, a boy named Fathom, assimilate, she begins to fall for him. But their love is a dangerous one, and there are forces on both sides working to keep them apart. Only, what if the Alpha are not actually the enemy? What if they are in fact humanity’s only hope of survival? Because the real enemy is coming. And it’s more terrifying than anything the world has ever seen.

Action, suspense, and romance whirlpool dangerously in this cinematic saga, a blend of District 9 and The Outsiders.