This is how it works: "When you start one of these swim workouts, the Watch auto-locks the touchscreen display. Apple says this is so the Watch won’t react to water droplets or water pressure as it would your finger. Before you hit 'End' on the workout, you have to twist the physical crown on the Watch. It then emits a sound, one that sounds uncannily like a mosquito flying close to your ear, and the vibration pushes water out the horizontal speaker holes."