The Incredible Tide — Alexander Key

A castaway on a rocky island is captured by a gang of evil men

He was born Conan of Orme, but Orme is no more. When nuclear war causes the oceans to swallow up the Western world, Conan escapes by chance, washing up on a craggy, desolate isle. After years of privilege, island life is a hard adjustment, but he grows strong -- learning to fish, to make fire, and to befriend the birds. On moonless nights, he screams into the darkness, tortured by a loneliness he cannot overcome. One day, a ship appears on the horizon, and Conan believes himself saved. But for this young survivor, trouble is just beginning.
 
The ship belongs to the New Order, cruel rulers who are rebuilding Earth through brute force. They send their new slave to the cutthroat city of Industria, intending to break his spirit. But Conan finds power on the island, and with it, he will remake the world.