If you own the cellular version of the Series 5, Series 6, or Series 7, you can call emergency services directly from your watch. The feature also supports international calls to emergency services, regardless of your carrier or where you live. So if you're an American visiting London, for example, and need help, the watch will call 999 and not 911.
Press down on the side button for a few seconds until you see the screen for Power Off, Medical ID, and Emergency SOS. You can either slide Emergency SOS to the right or continue holding down the side button. After a short countdown, your watch calls emergency services. If you start the countdown by accident, release the side button. If you start an emergency call by accident, press down on the screen and then tap End Call.
As an added feature, you can add an emergency contact through the Medical ID screen in the Health app on your iPhone. That person then receives a text with your location after the emergency call ends.