David Hakimian, 58, an oncologist from the Chicago area, was looking forward to a week of lazy days on Caribbean beaches when he boarded Windstar’s 312-passenger, recently stretched-out Star Breeze-only to find he’d instead be spending a week almost entirely at sea. In Iceland, passengers on the Viking Sky consoled themselves with meals of lobster Thermidor and Dover sole after similarly being turned away from nearly every port they intended to visit. And in Alaska, some sick passengers and crew were stuck quarantining in the fishing town of Petersburg after a 10-person coronavirus outbreak ended a 10-day sail on the 175-passenger American Constellation.

Cruise companies have had more than 15 months to iron out their pandemic-era operations — longer than any other industry. But after seven major lines resumed operations in the U.S. this summer, it has become immediately clear that this ever-changing virus can scuttle even the best-laid plans.

Their executives, who had generally accepted the reality that some COVID-19 cases would crop up on their ships no matter what, have found themselves up against two unexpected complications: port towns with zero-tolerance policies and the looming threat of the delta variant.