Mr. Kalmann grew up one of four boys in the Netherlands. At 19, he flew to New York for a theater internship and adventure. It was July 1973, and almost immediately, he got into an argument with a taxi driver; he relished every moment of it. Within a month, he recounted, he met John Lennon outside Phebe’s Tavern on the Bowery. He turned 20 that day and the Beatle wished him a happy birthday, he said.

A few years later, on a camping trip, he came upon three brothers thrashing in a river, clearly distressed. Mr. Kalmann jumped in and pulled two of the men to shore. The third slipped under the black water and disappeared. He dove back in again and again, he said, but never found him. “I tried very hard,” he said. “I just didn’t have the skills.”

That day haunted him. He stayed out of the water for about a decade. The memory stayed dormant until his sister-in-law’s comment took hold of him. “I want to do right by what I couldn’t have done that time,” he said.

Lifeguard hopefuls pass a qualifying test to secure a spot in the four-month training program to prepare them for the final tests. For about three hours every week, they learn water rescue techniques, first aid and C.P.R. It was there that Mr. Kalmann met Mary Jacobus, a 65-year-old public school social worker who was retiring in July, and Liang Sung, who, at 66, was looking for something to keep him busy after losing his Chinese-language news job in 2020. They trained alongside 81 others, many of them teenagers. All the workouts and training in the pools proved exhausting.

“We’re really old to do this job,” Mr. Sung said.

Initially, Ms. Jacobus’s friends warned her that it was too dangerous and pointed out that she would be working more hours as a lifeguard than she did as a social worker. But like Mr. Kalmann, she felt a deeper calling. She is the daughter of a Lutheran pastor and grew up doing volunteer work before entering social work. “I just really wanted to be helpful,” she said.