“We could potentially see a truly outsized spring break at a time when the last thing we want are major gatherings,” said Miami Beach Mayor Dan Gelber, a Democrat. He cited public-health specialists’ concerns that such crowds could generate another surge of Covid-19 cases.

Bars, restaurants and clubs will be open at no less than 50% capacity, because the city can’t shut them down under a state executive order. But Miami Beach—a barrier island known for its stylish hotels and spirited nightlife—is taking a zero-tolerance approach. Officials have beefed up restrictions from Feb. 22 to April 12 and expect peak activity in March. They have also launched an ad campaign urging young people to vacation responsibly.

More police officers and code-compliance staffers are patrolling to enforce measures including a midnight curfew and bans on alcohol and boom boxes on the beach. Music at venues can’t exceed ambient noise levels. Beach patrols are ensuring groups maintain social distance and wear masks when appropriate.

With all the limitations, many people are moving the festivities to party boats, officials say, so the city is clamping down on those that lack proper licenses or violate noise restrictions.