Esther Williams, a championship swimmer and lustrous beauty who became one of the world’s most popular movie stars in the 1940s and ’50s by appearing in aquatic musicals featuring daredevil plunges from pedestals, trapezes and even a helicopter, died June 6 at her home in Beverly Hills, Calif. She was 91.

Her longtime publicist, Harlan Boll, confirmed her death but did not cite a specific cause.

A California-born model who held a national record for the 100-meter freestyle, Ms. Williams was 19 when she was hired by Metro-Goldwyn-Mayer in 1941. She was the studio’s response to Sonja Henie, an Olympic gold medalist in figure skating who became a box-office powerhouse at rival 20th Century Fox.

“Melt the ice, get a swimmer, make it pretty,” studio chief Louis B. Mayer commanded. The result was Ms. Williams.

After small roles, she made her star-making debut in “Bathing Beauty,” released a month after the June 1944 Allied invasion of Normandy. MGM promoted her as “the girl you will dream about!” and she became one of the leading pin-ups among American servicemen during World War II — all 5-foot-8 of her lounging poolside in alluring poses.

For his new star, Mayer ordered a $250,000 pool with special-effects equipment for plumes of fire, underwater geysers and a central pedestal that, powered by a hydraulic lift, rose at least six stories above the water. “Never had plumbing been put to a more glamorous use,” Ms. Williams observed decades later in her memoir, “Million Dollar Mermaid.”

In the next 15 years, Ms. Williams’s irrepressible smile, physical allure, athletic grace and sheer stamina made her one of the most bankable stars of the era. She headlined aquatic extravaganzas and light romantic comedies with titles such as “Jupiter’s Darling” and “On an Island With You.”

Her co-stars ranged from Gene Kelly and Frank Sinatra (“Take Me Out to the Ball Game”) to the animated cat and mouse Tom and Jerry, who rescue her from a grabby octopus in “Dangerous When Wet.” Ms. Williams and Ricardo Montalban sang the Oscar-winning Frank Loesser duet “Baby, It’s Cold Outside” in the 1949 musical “Neptune’s Daughter.”

Ms. Williams defined a wholly new genre of film — Technicolor “aqua ballets” — that has never been imitated. The extraordinary showmanship of Ms. Williams’s movies, featuring dozens of acrobatic and precision-swimming routines, was later credited with popularizing the future Olympic sport of synchronized swimming. She was presented to audiences as a modern-day cross between Aphrodite and Venus: performing a ballet underwater to “Dance of the Reed-Pipes” from Tchaikovsky’s “Nutcracker Suite,” emerging into view from plumes of colored smoke and leaping into water from a trapeze, or accompanying many of the world’s top water-skiers in geometric formation before being hoisted 80 feet in the air by a helicopter for one final plunge.

At the time of the helicopter dive, from her 1953 film “Easy to Love,” Ms. Williams was pregnant with her third child. She did the water skiing, despite no previous experience with the sport, and agreed to be lifted into the air. She requested only that the studio hire a professional diver for the climactic drop.

She estimated that her eardrums burst at least seven times during her movie career. Possibly the most terrifying stunt she performed involved a 60-foot dive on the set of “Million Dollar Mermaid,” the 1952 biopic of Australian-born swimming star Annette Kellerman.

For that movie, the costume designer fitted Ms. Williams’s swimsuit with 50,000 gold sequins that weighed her down like chain mail, the actress wrote in her memoir. Tethered to her hair was a “gold crown” made of aluminum instead of a more pliable material such as cardboard.

The hydraulic pedestal lifted her into the air. The director, Mervyn LeRoy, yelled for her to jump, and Ms. Williams did, then she heard a snapping in her neck as the crown smacked the water.

She had seemingly performed the jump to perfection. LeRoy called, “Great . . . time for lunch!” and the crew scattered. Only the wardrobe lady heard the actress’s cries and rushed to find assistance. Ms. Williams had cracked three vertebrae and spent six months in a full body cast.

Ms. Williams was never under the illusion that her films were designed to win Oscars. She gradually made peace with her position in the studio’s economic hierarchy. She once told Deborah Kerr, the elegant, English-born actress who specialized in prestigious dramas, “If I make one ‘Neptune’s Daughter,’ you can make two ‘If Winter Comes.’ ”

Esther Jane Williams was born Aug. 8, 1921, in Los Angeles, and was the youngest of five children. In her memoir, she described a traumatic childhood that included the loss of a beloved older brother from a burst colon.

Her family then took in a surrogate son, a homeless 16-year-old boy who Ms. Williams said repeatedly raped her starting when she was 13. Her parents, she said, partially blamed her when she told them after two years.

She found refuge in swimming at her community pool. By 15, she was invited to join the prestigious Los Angeles Athletic Club, and she established records in national women’s swimming competitions.

When the 1940 Olympics in Helsinki were canceled because of the war in Europe, she turned to department store modeling. She also appeared with former Olympic swimmer and “Tarzan” star Johnny Weissmuller in Billy Rose’s Aquacade show at the 1940 San Francisco World’s Fair.

She was soon recruited to MGM. Mainstream critics routinely dismissed Ms. Williams’s films as trifles usually involving contrived plots of mistaken identity. Reviewers found her appealing to the eye but called her out of her depth when not in the pool. And at times, her movies stretched the credulity of even her greatest fans, such as when the studio cast her as a female matador in “Fiesta” (1947).

Her last MGM musical was “Jupiter’s Darling” (1955), which underperformed at the box office and gave the studio an excuse to show her the door. Meanwhile, she was busy managing a difficult private life.

Her first marriage, to Leonard Kovner, a doctor, ended in divorce. In 1945, she married Ben Gage, a radio announcer and singer. Ms. Williams had three children with Gage, whom she described as a compulsive gambler and alcoholic who sponged off her earnings. She eventually discovered that his bad business decisions left them $750,000 in debt to the Internal Revenue Service (she gradually paid it off).

In 1959, a judge granted her request for a divorce when she told him, “Your honor, I’m really tired of being what my husband does for a living.”

In 1961, she became the companion of her former “Dangerous When Wet” co-star Fernando Lamas, an Argentine-born actor with a dictatorial streak. She assented to his demands: giving up her profession, devoting herself to his career and cutting ties to her children.

To Lamas, she wrote, the children were a reminder she was not a virgin when they met. For years, she secretly cooked for her kids and drove them food, only to creep back home to Lamas.

In return for her sacrifices, she wrote, Lamas said he would renounce his well-known playboy habits. “Marriage to Fernando offered shelter and security, but the shackle was the price I’d pay,” she wrote in her memoir.

After Lamas’s death in 1982, Ms. Williams reemerged to public life, selling her own line of swimwear and reuniting with her children. In 1994, she married actor and producer Edward Bell.

Besides her husband, of Beverly Hills, survivors include two children from her marriage to Gage, Benjamin Gage Jr. and Susan Beardslee; a stepson from her marriage to Lamas, the actor Lorenzo Lamas; two stepchildren from her marriage to Bell, Tima Alexander Bell and Anthony Bell; and eight grandchildren. Her son Kimball Gage died in 2008.

When synchronized swimming was recognized for the first time as an Olympic event at the 1984 Games, Ms. Williams was feted as an unofficial godmother to the sport.

Watching the athletes, she later told Diane Sawyer of ABC News, “The only thing is, they didn’t smile. And I would say, you’re happy, you’re in the pool.”