Consider the New Year’s Eve festivities held on ships around the world. One day after the C.D.C. announcement, as dozens of crew members and other passengers were confined to small cabins, infected and quarantined, thousands of revelers packed into ship bars, casinos and deck venues, partying like it was 2019.

Harvey Freid, 56, recently returned from a 17-day sailing to Antarctica, during which one positive case was reported. But Mr. Freid, an avid cruiser, is undeterred and is scheduled to go on a Caribbean cruise in late January.

“The cruise ships do a very good job of handling Covid, and I think it’s safer than my building in Miami and most places that I go on land, because people are vaccinated and cases are quickly identified and isolated,” he said.

The cruise industry’s trade group, Cruise Lines International Association, called the C.D.C.’s warning “perplexing,” and said that cases identified on ships “consistently make up a slim minority of the total population on board — far fewer than on land.”