Juli Clover, MacRumors:

With the LTE-enabled Apple Watch Series 3 only available in a handful of countries at the current time, it might be tempting to purchase one in another country, but it won’t work. Series 3 models appear to be limited to connectivity in their original country of purchase.

An Apple support representative who spoke to MacRumors reader Thomas said that an Apple Watch purchased in the U.S. online store will only work with the four carriers in the United States, perhaps due to hardware limitations.

So no buying an Apple Watch in another country, and no using your Apple Watch cell when you travel internationally. Caveat emptor.

UPDATE: Did some reading on the Apple Watch carriers page. Seems to me it’s more country groupings, rather than individual countries. One model would work in US, Canada, Puerto Rico, another model in Australia, France, Germany, Japan, Switzerland, UK. Still worth knowing it, still suggests caveat emptor.

UPDATE 2: From this Telegraph article [H/T Sam Gross]:

The feature is likely to be a boost for runners who leave their phone at home, or when a battery dies. However, it has emerged it will only connect to networks in the country it was bought from.

This means owners in the UK and Europe, who ostensibly enjoy free mobile roaming, will not be able to connect when crossing borders.

And on the carrier page referenced above:

Roaming is not available outside your provider network coverage area. So that clarifies that.