Eliot swimming in Virginia, about six months before her arthritis symptoms emerged

My daughter, Eliot, and I are floating facedown in the Caribbean Sea a quarter-mile off the coast of Grand Cayman Island.

Our snorkel tips fight for air above the chop as we stare into the depths at a Greco-Roman warrior with a seahorse’s tail. A 13-foot-tall statue known as the Guardian of the Reef, it rises from a sand flat 65 feet below us. This is our final afternoon of a two-day free diving course, and it’s Eliot’s turn to go off-rope. A weighted guideline knotted every 16.5 feet, the rope has served as a focal point for our battery of trials. Having it at hand also made the ocean seem smaller somehow. No longer. The Guardian beckons Eliot with his raised scepter. I don’t want to let her go.

Except that I’m the one who brought her here. We came for the experience—to have fun—but when you’re father and daughter, life’s never that straightforward. Even less so when she’s 12, on the cusp of puberty, and literally transforming before your eyes, from a bright, honey-brown-haired, firstup-in-the-morning daddy’s girl to a moody, sarcastic stranger with breasts, hips, and the sleep habits of Dracula. Worse, my irrepressibly confident girl is suddenly taciturn and self-conscious. I hate the thought of fear holding her back. It reminds me too much of my own childhood.