" a registered but undocumented boat, US flagged."
Dubon, if it is undocumented, it is NOT US FLAGGED. It is only "flagged" by the State that issued the registration, and that's not the same as a national flag.
I doubt any country will deny you entry or ask for your insurance status, but you may find that marinas are reluctant to let you tie up without insurance. Countries don't care about that AFAIK
. Businesses like marinas do.
Got an itinerary? Any idea where you will be going? OK, email a couple of marinas and ask what their insurance requirements are, or whether they have any.