http://www.ryatraining.org/professio...dorsements.htm reads:
"The standard commercial endorsement allows you work on board British flagged vessels which are subject to the MCA’s codes of practice for small commercial vessels. The following wording would appear on your certificate:"
"For Yachtmaster® Offshore and Yachtmaster Ocean certificates of competence: “This certificate is valid for use as a Master of yachts of up to 200gt on commercially and privately registered yachts until (date of expiry)”
“The certificate holder complies with STCW 95 regulations V1/1 Sections V1/1-4. This certificate is valid for use as a Master of
yachts of up to 200gt on commercially and privately registered
yachts until (date of expiry)”
Yacht means yacht, a vessel with a certificate describing it as a yacht, holding a manning certificate (if applicable) describing it as a yacht.
A USCG 100 ton master can legally operate a yacht, a head boat, a ferry, or any other commercial vessel of not more than 100 tons. It is not restricted to yacht service. In real life a 100 ton is not worth much as it is not an internationally recognized license so the point is moot really. A 200 ton master with an oceans endorsement is about the minimum valid for international service if you want to swim in the yacht pool.