The news, when it reached the Grand Princess early on March 4, barely registered at first. In a letter slipped under passenger cabin doors, Grant Tarling, Carnival Corp.’s chief medical officer, announced that the U.S. Centers for Disease Control had begun “investigating a small cluster” of Covid-19 cases in California that might have been linked to the ship. Thirteen days after leaving San Francisco for Hawaii, the vessel would be skipping a scheduled stop in Mexico on its return voyage and sailing back early to its Bay Area port.