"This is paid once, and transfers to subsequent buyers, unless the vessel is sold to a buyer who registers outside of the US at any point."
this has not been my experience. Once a vessel has been imported and the duty paid, US custom will not impose duties against even if at some point the vessel was foreign flagged.
"Us Built vessels are not subject to this duty, again, unless they are registered outside of the US."
Customs duties are charged based on country of origin, i'm not sure how duties can be imposed on a US manufactured product, even if the vessel was foreign flagged for a while.