Ren Yu is a swimmer. Her daily life starts and ends with the pool. Her teammates are her only friends. Her coach, her guiding light. If she swims well enough, she will be scouted, get a scholarship, go to a good school. Her parents will love her. Her coach will be kind to her. She will have a good life. But these are human concerns. These are the concerns of those confined to land, those with legs. Ren grew up on stories of creatures of the deep,...