I thought it was a requirement that a non-US registered ship must stop at a foreign port, it can't simply sail between two US ports.
It actually isn't entirely registry, although registry is encouraged for regular loops, as is building the ship in the US, although some companies have gotten waivers there. If a ship cruises exclusively between US ports, as with a Hawaiian cruise that does not go from Vancouver or Ensenada or detour to Micronesia, the crew must be majority US citizens and comply with US labor laws.
This is also why Alaskan cruises always start from or stop in Canada somewhere. It allows them to skirt the labor rules for US-only cruises.
I think that it can do this if the beginning and ending port are the same port and it also hits another port in between. But if it's two different ports for start and end, that's definitely a no-go. Though it may also be true that you have to hit a foreign port in between even for the same start and end port. I know that they've recently ruled that cruises to nowhere that simply sail out and return to the same port can't be done.
I also know that the cruise line can be fined if someone leaves the ship in a US port after starting in a US port (because they start and end in two different US ports). You also can't, for example, board in Key West if you miss embarkation at another US port - you have to board in a foreign port.
Daft rules, if you ask me, but that's what it is. Also worth nothing that you have to be a US citizen to be able to not have to use a passport for a closed-loop cruise even if you are a US resident.
Just some clarification - this doesn't really have anything to do with passports or not, but the issue of what itineraries are legal.
The law (Passenger Vessel Service Act) states that a foreign- flagged ship may not transport passengers from one US port to a
different US port with out a stop in a distant foreign port.
A
distant foreign port is described as any port NOT in North America, Central America, the Bermuda Islands, or the West Indies (including the Bahama Islands, but not including the Leeward Islands of the Netherlands Antilles, i.e., Aruba, Bonaire, and Curacao).
So, a cruise from, say Port Canaveral to San Diego, must stop somewhere other than those locations.
Any cruise that embarks a passenger in one US port, and debarks the same passenger in a different US port, without a distant foreign port stop, subjects the ship to a $300 fine per passenger. That fine is often passed onto the passenger.
A closed loop cruise (starts and ends in the
same US port) only requires a stop at ANY foreign port.
Regarding the passport on the Miami to San Juan cruise, from the
http://www.us-passport-service-guid...ruise-from-puerto-rico-to-new-jersey.html(not a government agency, however):
Since Puerto Rico is a territory of the United States, U.S. citizens are not required to present a passport when traveling between Puerto Rico and the United States. It is the same as if traveling between two states in the U.S.