Originally Posted by svzephyr44
your ham license is portable - wherever you are you can operate.
This is not true. The United States has agreements with a goodly number of countries that allow you to operate with just your U.S. license. There are, however, countries that do not have such an agreement, and transmitting on the ham bands from inside their jurisdiction, without first getting a reciprocal operating permit from them, would be illegal. A nearby example would be the Bahamas.
Here is an ARRL web page with more information...
Reciprocal Permit FAQs
Of course, in international waters you are not in anyone's jurisdiction. In that case your U.S. license is all you need.