Unlike many places in the world, we still have a bit of freedom left in America. US flag yachts that do not carry passengers for hire under 300 GRT (about 750 GT) do not require any inspections, or licensed or certificated personnel. Anyone can operate them with no certificates.
I believe that only the Master must be a US citizen. Legally, when in the US any foreign crew must have Green Cards or US crew visas. There is a limit on how long foreign crew can work in the US on crew visas. As I vaguely recall , once the US flag yacht goes foreign, it can hire all foreign crew except the Master.
In the past it has been very common for foreign crew who could not get visa renewals to work on US yachts illegally. I recall one blond, blue eyed Brit who had to leave when his visa expired, but returned by walking across the Mexican border and was back onboard within a week.
US flag yachts can be built anywhere, and many of them are, or they can be reflagged from foreign flag.
The Jones Act only applies to vessels engaged in the coastwise Trade. Yachts are not engaged in trade. I believe it is fairly easy get Jones Act waivers to carry up to 12 passengers. I’m not up to date on this.