Can a Apple Watch purchased in Canada be used by someone in the USA?

Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

The cellular models are regional. Make sure what you buy works in the country you are taking it.

Be aware that some features are restricted based on where the iPhone was purchased and/or where it is being used. The most restrictive is ECG. Both the country of purchase and the country where used have to have given government approval.

Apple Watch Feature Availability by country

https://www.apple.com/watchos/feature-availability

Technically the GPS+Cellular watch model is the same model for the U.S. and Canada so it should also work, but there have been several instances where carriers in the U.S will not activate the watch on their cellular service if it originates outside of the U.S, and sometimes even watches from the U.S but bought from a different carrier can cause issues. Verizon is notorious for this, and there are tons of posts about Verizon beng unable to activate the cell service for a watch not meant specifically for them.

Other's like AT&T and T-Mobile seem to have very few issues when activating the watch.

Without cell service active, the GPS+Cellular watch will act like a regular GPS-only watch, so it will still work for the most part.

You can see here for specifics: Use your Apple Watch without your iPhone nearby - Apple Support

Yes. The GPS model is universal, and can be used anywhere in the world. The iPhone's carrier does not matter either since the GPS only Apple watch connects directly to the iPhone not the carrier.