If you're searching for ways to beat the heat in Boston, Mayor Marty Walsh has some good news: On Wednesday, the city is opening two outdoor swimming pools to the public.

As is often the case in the COVID-19 era, though, there's a catch. Several, actually.

Before visiting the Clougherty Pool in Charlestown, or the Mirabella Pool in the North End, users will be screened twice for COVID symptoms — once when they register for a session online at boston.gov/bcyf/summer, and once when they arrive at the pool. Advanced registration is required, and available 24 hours in advance.

Swimmers will also have to wear masks when they're not underwater, stay 6 feet away from other household groups, and leave the water after 90 minutes. The total number of people allowed in each pool will be limited to 40 percent of full capacity — which means, Walsh said in a press briefing Tuesday, 75 people or less.

"A lot of hard work has gone into our pools' safely, and we ask for everyone's cooperation to make this plan a success," Walsh said.

The good news, beyond timely expanded access to cooling waters? In contrast to past years, admission is free.

Walsh also had some pointed advice for the people who thronged Boston's public beaches during the recent heat wave: mask up, and spread out.

"We saw pictures this weekend of the beaches in South Boston where that was not happening," Walsh said.

According to Wash, though, there's no reason for beachgoers to pack together.

"On the beach in South Boston, those that go there, many people sit on the M Street beach side," he said. "If you just go to the other side of the bathhouse, there is about a mile of beach that no one's using. You can go there and spread out and physically social distance and really be safe, and keep yourself safe, quite honestly."

That exhortation came after Walsh delivered good news about some key COVID-19 metrics in the city. In the week ending July 13, Boston's positive COVID-19 test rate dropped to 2.1 percent, down from 2.5 percent the previous week. In addition, the city reported just 4 new cases Monday, and there were no new fatalities.

"Our job is to keep that trend going in the right direction," Walsh said.

"There is a clear cause and effect relationship between collective decisions, individual behaviors, and the spread of the virus."