The headlines on Sunday about a Norwegian Cruise Line ship sailing in the Gulf of Mexico were alarming. There was an “outbreak” of COVID-19 taking place on the brand’s giant Norwegian Breakaway, as many news outlets put it, and the vessel was just hours away from docking in New Orleans. To a casual observer, it may have seemed like we were right back to early 2020 when the just-emerging coronavirus was running rampant around cruise ships, prompting panic in port towns, quarantines on ships and the sight of ambulances meeting vessels as they docked. But this isn’t early 2020. As...