What Dawn said about United States lines is accurate -- the reactivated line (which was the original owner of the SS Constitution and the SS Independence, both subsequently acquired by American Hawaii Lines) bought the MS Nieu Amsterdam from Holland America Lines for refurbishment and and service as MS Patriot, offering cruises that operate round trip from Honolulu with ports of call at four other islands of the fiftieth state. United States Lines also ordered two new cruise ships from Litton Ingalls Shipbuilding in Pascagoula, Mississippi. The new vessels are the largest passenger vessels ever built in the United States and the first ocean-going passenger vessels built in the United States in about forty years.
What Dawn did not say is that American Classic Voyages, the parent of both American Hawaii Line and Delta Queen Steamboat Company, bought the rights to the name "United States Lines" for what apparently will be a more upscale operation than the present American Hawaii Line. Thus, it's really operated by the same company.
You should not expect to see any other cruise lines operating seven-day cruises within the Hawaiian islands. There's a serious complication to their doing so. A federal law called the "Jones Act" requires that all vessels transporting passengers from one port within the United States to another must be of U. S. registry -- which also requires U. S. crews whose powerful unions demand prohibitive wages. Thus, most cruise lines register their ships elsewhere to cut costs. Princess's vessels now are of British registry (the home port of MV Crown Princess and MV Grand Princess is Hamilton, Bermuda, and the home port of the rest of the Princess fleet is London, England) and some Holland America vessels may still be registered in the Netherlands Antilles, but most other major cruise lines register their vessels in Liberia, Pamama, or the Bahamas for purely economic reasons.
Anyway, under the Jones Act, vessels of foreign registry cannot legally transport passengers from one port within the U. S. to another unless they stop at a "distant foreign port" (Aruba, Bonaire, and Curacao and the continent of South America qualify) en route -- which is why the ships leave from Ensenada rather than San Diego when they are going to Honolulu and vice versa. They can return passengers to the original port of embarkation if either (1) they have not called at any other port or (2) they have called at any foreign port en route -- which is why most Caribbean cruises return to the original port of embarkation..