Within a very short time after leaving a port, the cruise ship is in international waters, and no longer in the United States.
So, any coverage by those who have coverage in the United States will not cover any treatment necessary on the ship.
Bottom line... for the minimal cost of travel insurance one has to be a FOOL not to purchase it.
Of note, the insurance available from the cruise lines does not necessarily offer you the coverage you need. Look at third party suppliers for less expensive and more thourough coverage.