With an ordinary heart rate sensor, you wouldn't jump into the pool. But the Garmin HRM-Swim was developed especially for swimmers. The heart rate belt won't get damaged by the chemicals in the water and it features an anti-slip design to ensure it'll stay tight around your body.
Note! There are only a few sports watches that can register heart rate data during swimming: the Garmin Forerunner 920XT, Garmin Epix, and Garmin Fenix 3.