Apple Watch is resistant to splash and contact with water, but not waterproof. For example, you can take it when you do sport, when it rains or when wash you your hands, but not recommended for use under water. Apple Watch has a grade according to the standard IEC 60529 IPX7 water resistance. Skin belts are not water resistant.
With this note at bottom of page, Apple warns Apple Watch its degree of resistance to the water and the type of situations in which we may or not have problems in the section on health and fitness. The curious thing is that precisely the degree of protection IPX7 requires that the device supports without any filtration the complete immersion to 1 meter deep for 30 minutes.
Of course it is not the degree of resistance that we would for a watch as the sport-oriented, although it is definitely more than expected considering the previous sitting by the iPhone to date against a competition full of submersible smartphones.