On a hot weekday morning in late July, I arrived at Marcus Garvey Pool in Manhattan for a long-awaited meetup with a mermaid. Cookie DeJesus, also known as the Harlem Mermaid, has become a fixture in those chlorinated waters over the past several years, ever since she took a certification course at World of Swimming, a mermaid school in Brooklyn, and fell in love. Most days, she and her husband, Ralph, who designs many of her aquatic costumes, walk from their apartment to the pool so DeJesus, 57, can do laps in full mer-regalia as part of her workout.