Apple doesn’t advertise the Apple Watch as being waterproof or rugged. But the device can last a little while when exposed to water. This video from DC Rainmaker shows an Apple Watch being tested in a waterproofing chamber to 40 meters.
As you can see in the above video, the Apple Watch can survive underwater for a while. Of course, Apple is not going to tell you that, so if something goes wrong, you are on your own.