Exodus by Bertagna

The year is 2099. The polar ice caps have melted and the world is flooded. Mara Bell lives with her family on one of the few remaining islands. But even their land isn't safe. The sea continues to rise and soon the people of the island must make a choice: move or die. Mara does some research and discovers that before the great flood, huge "New World" cities were built like trees into the bedrock of the earth below. In their synthetic branches, civilization continues. Mara convinces her people to seek out the New World, but finding it turns out to be the easy part. The challenges that follow their arrival test everything Mara knows about herself and the world around her.

I was expecting a horrible rehash of Waterworld. I was very pleasantly surprised. The book is thoughtful and brings up some interesting issues about immigration, prejudice, and what it means to stay human.