Recent reports of COVID-19 infections aboard cruise ships — along with simple math — suggest that it will be nearly impossible for any cruise line to return to the seas without the coronavirus lurking. All vaccines are less than 100% effective, so chances are strong that at least some passengers will be infected among the thousands aboard even “fully vaccinated” voyages. Chances of more severe outbreaks are even greater in light of a court ruling Friday in favor of Gov. Ron DeSantis, who wants to stop cruise lines from requiring vaccinations. Cruise lines faced with infections will have to perform...