I wrote this for iPhones but many things probably apply to watches too:
Buying an iPhone in one country to use in another country can have many problems:
LTE Find the right iPhone for the right country or region. - http://www.apple.com/iphone/LTE/
E.g., Using American iPhone 5 in Europe - https://discussions.apple.com/thread/4593901 - Yes, but no LTE support.
Warranty restriction! - http://www.apple.com/legal/warranty/products/ios-warranty-document-us.html - "IMPORTANT RESTRICTION FOR iPHONE, iPAD AND APPLE TV SERVICE. Apple may restrict warranty service for iPhone, iPad and Apple TV to the country where Apple or its Authorized Distributors originally sold the device." <-- Read this as they almost certainly will refuse to do warranty coverage except where you bought it. The phone would have to be sent to a friend in in the country of purchase to handle any repair claims because Apple will not handle international shipping.
New phones may also have tax added to the cost (e.g., in the USA prices do not include taxes), and you may have to pay import duty when bringing an expensive phone into your country.