The country of registration is separate from the country the ship was built in, and separate from ownership or nationality of the crewmen. A ship can be built,

The country of registration is separate from the country the ship was built in, and separate from ownership or nationality of the crewmen. A ship can be built, owned and operated from outside the US but still be registered in the US. You may be thinking of the Jones Act, which implements cabotage in the US, a fairly common practice among seafaring nations: https://en.wikipedia.org/wiki/Cabotage https://en.wikipedia.org/wiki/Cabotage