In a swimming pool in Italy, ten competitive swimmers exposed a quiet failure at the wrist: the optical sensors that guide millions of people through their daily fitness routines cannot see clearly through water. What works on a morning run dissolves into unreliable numbers the moment a body enters the pool, revealing that the physics of light and water have always been in tension — and that wearable technology, for all its promise, has been designed for a world that stays dry. Researchers have now documented this gap systematically and opened their data to the scientific community, framing a