To gain a USCG 6 Pax license you take theory tests and document boating time on the water since you were 16 years old. There is not necessarily any sailing experience documented. Even if you have the sailing endorsement, this still does not replace a sailing resume which is required for all charter companies.
In essence - no matter what you have you must have a sailing resume to charter a yacht on a sailing vacation. Here is more about a sailing resumes.
So a USCG license gets the attention of a yacht charter company but a sailing resume is still required.
Specifically for the Mediterranean, the licenses valid for recreational operation in the Mediterranean require to holder to have done an assessment of their skills on a vessel of similar size and type. The USCG license has no such assessment requirement. While you may be competent and the driver of a big tanker, other people holding a USCG license may have never been assessed and really not very competent. So the license is clear - an assessment of skill is required.