Passenger Vessel Services Act of 1886
SIMPLY EXPLAINED:
A foreign flagged vessel cannot transport passengers from one US port to another. There must be a stop in a foreign country during the trip to avoid violating this act.

Missed your ship? Can you board in another port?
Even in cases where it is logistically easy to get to a ship's next port of call, you may not be legally allowed to board the vessel at that port. Some countries, including the United States, have "cabotage" laws that restrict the number of ports where foreign-flagged vessels such as cruise ships can pick up and drop off passengers. These laws are designed to protect domestic shipping industries from foreign competition.
In the United States, the Passenger Vessel Services Act of 1886 says foreign vessels cannot transport passengers from one U.S. port to another U.S. port, either directly or by way of a nearby foreign port (defined as ports in North and Central America, Bermuda or most Caribbean islands). The Act does not apply to sailings that begin and end in the same U.S. port, provided the sailings include at least one stop at any foreign port. It also has an exception for trips between two different U.S. ports that include a stop at a distant foreign port.
What that means, in practice, is that a cruise line cannot let you join a ship at a U.S. port such as Port Canaveral, Florida, if you later will be disembarking at a different U.S. port — say, New York or Miami — unless a visit to a distant port such as Aruba is on the itinerary. This is a rule that can severely limit your options if you want to join a ship part way through a cruise or have plans to disembark early.