As an American, to use a VHF Marine
radio abroad you are suppose to be licensed by the FCC, in most cases that means a Ship Station License.
The problem as I see it is that the FCC hasn't caught up wth new technology... FCC currently states you are suppose to use the same MSSI number that is assigned to your vessel's Ship Station License on all portable VHF radios on the boat.
They don't address the problem that a hand held VHF with DSC might not be on the boat it is assigned to, for instance you crew on another boat, but take you hadn held with you... Or in you case, own a VHF Marine
Radio, but no boat...
So relistically if you register with Boat US and put the MMSI number in the radio, no one is going to ask questions in the US or abroad... As long as you use the radio responsibly