Here is an excerpt from the Passenger Vessel Service Act:
Transportation of Passengers Between Puerto Rico and Other U.S. Ports—46 U.S.C.§ 55104
An exception to the PVSA permits non-coastwise-qualified vessels (vessels not qualified to engage in the coastwise trade) to transport passengers on voyages between ports in Puerto Rico and other U.S. ports until qualified U.S. vessels are available.
You can get a pdf of the law here. There is also an exception for Canadian vessels to service Rochester and Alexandria Bay, NY.