46 CFR 90.05-1(a) creates a wierd situation where US flagged seagoing motor vessels over 300 gross tons (even recreational vessels not engaged in trade) could be required to hold a USCG issued Certificate of Inspection (COI). But it also states in 90.05-10 that this requirement does not apply to yachts on an international voyage...unless of course the vessel is carrying passengers for hire (that's a different class of vessel).
Our laws and regulations (USCG and CRF's) are applicable to any foreign vessel operating in US waters. So, the requirements of 46 CFR 90.05-1(a) would apply to a foreign flagged vessel of more than 300 gross tons and carrying 12 or fewer "passengers"...unless that foreign vessel is flagged by a nation that is signatory to SOLAS and has onboard a current and valid Safety Equipment Certificate. "Passenger" is further defined in 90.10-29...and should not be confused with passenger for hire. Anyway, SOLAS I/A Reg 3 exempts all pleasure yachts not engaged in trade, as well as Cargo ships of less than 500 gross tons. Cargo Ships are permitted to carry 12 or less "passengers", and a Passenger Ship is specifically defined as a ship which carries more than 12 passengers - SOLAS I/A Reg 2. Cargo Ship of over 500 gross tons would be required to have a Cargo Ship Safety Equipment Certificate, and a Passenger Ship (of any gross tonnage) would be required to have a Passenger Ship Safety Equipment Certificate.
For enforcemtent of US Laws/Regs and SOLAS on the waters of the United States there exists a loop-hole in the rules whereby a foreign flagged recreational yacht between 300 and 500 gross tons, which operates on international voyages, and carries 12 or less passengers is required (by US regulation) to have a certificate that doesn't exist (the Safety Equipment Certificate). Working with other flag administrations (like Bermuda) we have found that they do not issue SOLAS certificates to vessels that are not required to be provided under SOLAS. It's a wierd one...and between you and me...CG HQ doesn't want to clarify the regulations on this "minor" issue. So it really goes unenforced (probably because there is no way to enforce the regulation, and there is no way for a foreign flagged vessel of this type to seek an appropriate remedy in the way of obtaining the non-existent certificate).
What some foreign flagged yacht owners have done is registered themselves with their flag administration as a Cargo Vessel. That way they receive the appropriate SOLAS documents/certificates, and as a Cargo Vessel they are authorized to carry 12 or less passengers.