Sean Fittz; triathlete designed interiors for restaurants and homes in Boston, 40

It was mid-or-late afternoon, when he parked near the beach and talked on the phone with his brother Joseph of Charlestown. And then, he took the plunge.

“Sean was on his own practicing,’’ his brother said, for the Swim for Life in Provincetown in September, a swim that raises funds for local health services that he had done many times. That was just one among his many charitable acts.

On Tuesday, day passed into night and Mr. Fittz, it was later learned, did not return to his Back Bay residence. Anxious family and friends began calling one another. “Nobody knew until the second day around noon that Sean was missing,’’ said another brother, Charles of Andover.

State and local police were notified. Mr. Fittz’s car was discovered the next day. His body was recovered Thursday morning. He was 40.

The medical examiner’s report lists the cause of death as accidental drowning. Foul play was ruled out, Joseph said. The family is awaiting an autopsy report to determine the exact cause of death.

Mr. Fittz’s fulltime work was as an interior designer. Some of his venues were restaurants and more recently upscale residences.

Family and friends who knew of Mr. Fittz’s lifelong dedication to swimming and physical fitness, the many triathlons he took part in for different causes, his scuba diving and his participation in the first Boston-to-New York AIDS bicycle ride in 1995 to support AIDS research and medical services, were stunned by his death.

Susan Charbonnier of Charlestown has known Mr. Fittz all her life. “We swam on the boys’ and girls’ swim team in school in Charlestown since we were six or seven,’’ she said. “We continued swimming together through high school.’’

Much of the swimming in his adult life was to raise funds for charity, she said. “Sean was a fantastic swimmer. He swam all strokes but competed in the butterfly stroke. He has swum with my kids. His passion was travel, and he has thousands of friends all around the world.’’

Mr. Fittz looked like a swimmer. “He was very fit and very, very handsome,’’ Charbonnier said. “He used to go to the Ultimate Boot Camp on the Boston Common to stay in shape. He played golf with his brother, Joe. Sean was very kind, thoughtful and energetic, very happy-go-lucky and wonderful to his family and friends.’’

Sean Alexander Fittz was born in Boston and raised in Charlestown. He served as an altar boy at St. Mary’s Church in Charlestown. He graduated from North Cambridge Catholic High School in 1987 and attended Boston University for two years.

He began his career in the restaurant business as a busboy at the Blue room in Kendall Square. “Over the course of a decade,’’ Charles said, “he worked closely with such chefs as Todd English, Barbara Lynch, Chris Schlesinger, Paul O’Connell and Stan Frankenthaler at such restaurants as Olive’s, East Coast Grill and No. Nine Park.’’

At many of them, he worked his way up from busboy to redesigning the restaurant.

For 11 years until the time of his death, Mr. Fittz was the owner and operator of Sunfish Paint and Design, Charles said, “working with some of the city’s top designers on both commercial and residential projects.’’

He also found time to travel around the world.

Michael Carter, a Boston interior designer and a longtime friend, marveled at Mr. Fittz’s energy. “For someone age 40, Sean outlived us all,’’ he said. “He had a wonderful sense of humor and was extremely upbeat. He was sort of the unofficial mayor of the City of Boston.’’

Joe Antoun of Boston was Mr. Fittz’s high school math teacher, and they kept in touch over the years. “Sean always had a huge capacity for generosity, kindness, and forgiveness,’’ he said.

His travels took him through Europe, South America, Asia, and Australia, where he learned to scuba dive. Lisa Kerr, a Boston artist, was one of the lucky ones who traveled with Mr. Fittz. About five years ago, they went for a month to the African country of Ghana, under a Rockefeller Foundation program, to teach small business owners to market their crafts.

“Sean had such a gifted eye,’’ she said. She recalled how with a can of paint and other items he transformed a rundown African hut to an attractive shop for the owner’s crafts.’’

“Sean brought joy to everyone he met,’’ she said. “He lit up a room.’’

In addition to his two brothers, Mr. Fittz leaves his mother, Mary A. (Dolly) Hudson, of Charlestown; his father, Charles S. of Albuquerque; and a third brother, Liam, of Charlestown.

A funeral Mass will be said at St. Mary’s Church in Charlestown at 10 a.m. today. Interment will be private.