I read this on Mashable: "However, before you go diving off the deep end into the nearest pool, understand that the Apple Watch Series 2 is not ready for swimming all the time. Before going for a dip, you need to open the Workout App and select “Open Water Swim” or “Pool Swim.” This basically prepares the Apple Watch for going under the water (up to 50 meters)."
Source: http://mashable.com/2016/09/14/apple-watch-series-2-review/#EsGGhO6TQ8qW
Source: http://mashable.com/2016/09/14/apple-watch-series-2-review/#EsGGhO6TQ8qW