US documentation (documented vessel status) is reserved for US citizens partly because of traditional and commercial
provisions that gave the owners and crew special status and protections AS CITIZENS. I know some of these have changed or been dropped over the past 20-30 years but going back that long ago, for example, if you were crew aboard a US documented vessel and that vessel was stranded in a foreign port, the US government
would repatriot you, i.e. bring you home. I'm afraid I've lost
the notes I once had about specifics, these days documentation for pleasure boats seems to be mainly an issue of title registration.
On other perq for documented vessels is that the captain
may act as a defacto agent of the US postal service: If you have picked up mail in a foreign port and are bringing it inbound to the US for delivery
to the USPS (who does not need to authorize you for this) you are legally allowed to BRING THE MAIL. You are not required to stop for the USCG, Customs, etc, but are allowed to bring the mail in without delay. An obscure provision in the USCode which, I am sure, the nice men
in the striped boats neither know nor care about, I don't suggest doing this. But the odd little quirks like that are why documented status is reserved for US citizens and corporations that are more than 51% owned by US citizens, IIRC.
In the meantime, Peter, in order to make sure you don't run afoul of some petty inspector along the way, you might write ahead to the places you plan to visit, and get their written reply stating that there should be no problem, or what papers they want you to bring. If nothing else, when you present those along with your registration, it assures the locals that you have planned ahead and you are a legitimate visitor.