Before beginning her 28-mile swim around the island of Manhattan, Julie Billingsley asked a friend who had done it in 1978 just how nasty the rivers of Gotham really were.

"The way he described it, I thought I'd be swimming through a ton of debris," said Billingsley, a 24-year-old graduate student at Georgetown University. "But enough people had done it, and nobody's ever died, so I decided to give it a try."

Billingsley was one of 28 swimmers, four from the Washington area, who competed in the second annual 'Round Manhattan Swimming Marathon last month. That swim followed by two weeks a professional marathon off the coast of Atlantic City. And both came on the crest of a new wave of aquatic marathons that enthusiasts hope will soon become a full-bore, competitive craze.

The sport already has advocates who make die-hard runners sound like half-hearted wimps.

"After a marathon, I'm high for weeks," says Diddo Clark, a 34-year-old Washington attorney who gave up her legal practice this year to swim full time. Clark finished the first Manhattan marathon in last place after more than nine hours in the water. At Atlantic City last month, she battled a seven-knot current, breaking surf and jagged rockpiles for 12 hours and 20 minutes before being ordered from the ocean by concerned race officials. During the second Manhattan marathon, she became violently ill after breathing exhaust from the Staten Island ferry.

Sound gruesome? Clark hears only sweet body music. When she misses too many days of practice or tapers off before a big swim, Clark describes a physical reaction that an addict could relate to.

"Withdrawal from hard workouts makes me nervous, restless, irritable and depressed. I get aches and pains . . . I stand in the shower after a light practice and I can't remember whether I have just washed my hair."

Marathon swimming goes back a long way. Modern interest in the sport was aroused in 1915 when Robert Dowling was credited with completing the first swim across the English Channel. Attempts to duplicate that feat have become so commonplace that now only very old or very young swimmers get any media attention for their efforts.

Five years ago, marathon swimming got a boost from Diana Nyad, a New Yorker who seems as talented at self-promotion as she is at distance swimming. In 1978, with media from half a dozen countries following every stroke, Nyad attempted to swim from Cuba to Key West, protected by a $65,000 shark cage.

The attempt was foiled by high seas, jellyfish and faulty equipment. After 41 hours and 70 miles, Nyad was pulled from the water. But the front-page coverage, coming as it did during America's new obsession with marathon runs and triathlons, prompted a number of distance swimmers to abandon pools for the open sea.

Since then, Nyad has completed a swim from the Bahamas to Florida. Ionthe Rhodes, a 54-year-old woman from eastern Virginia, swam across the mouth of the Chesapeake Bay. Two weeks ago, Julie Ridge, a 26-year-old Broadway actress from Arlington, Va., became the first person to swim two laps around Manhattan. Because Ridge last appeared in "Oh! Calcutta!" a play famous for its nudity, New York newspapers had a field day with her story. One headline read, "Turns in Birthday Suit for Bathing Suit."

Distance swimmers have discovered a virtual new frontier for record setting. All they have to worry about are sharks, tides, errant propellers and the diseased condition of the water. But then getting into the Guinness Book of Records is not supposed to be easy.

"This is something I've never done before. I wanted to see if I had the mental discipline for it," said Art Smith, a 44-year-old computer systems analyst for the Department of Justice who competed in the second Manhattan marathon. What did his family and friends think of his decision? "They're amazed; and, to a certain extent, so am I."

Smith, Clark, Billingsley and Stacy Chanin, the four local swimmers in last month's marathon, all belong to the D.C. Masters swim team that is coached by John Flanagan. For 48 weeks each year, team members swim two to three miles a day at either the Northern Virginia Fun and Fitness Center or the East Potomac Park pool. Before the Manhattan marathon, the group practiced a few days in the Chesapeake Bay and the Atlantic Ocean.

But nothing could prepare them for the waters that swirl around Fun City.

The race began at the 89th Street pier, just below Gracie Mansion, at 10 a.m. The 28 swimmers, going counterclockwise around Manhattan, stroked north for half a mile of East River to Hell's Gate, where the East and Harlem Rivers meet. It was slack tide, the only time the whirlpools and four-foot waves there subside. After a seven-mile trip up the Harlem River, the swimmers crossed the northern tip of Manhattan at Spuyten Duyvil to the Hudson. For the 12 miles of Hudson River swim, the current was carrying them faster than most of the joggers who watched incredulously from shore.

The final leg of the trip was the trickiest. After rounding the Battery, the swimmers had to go back up the East River. If the timing is not right, a swimmer can meet an outgoing current that is impossible to buck.

The tide was supposed to go slack at 4:05 p.m. But because of recent rain, it did not occur until 4:20 p.m.

"We were on time, but the tide was late," said Clark, who recovered from her gas exhaust sickness in time to finish in 10th place. Billingsley finished fifth, and first among the women, in 8 hours and 33 minutes.

Chanin, a 22-year-old student at the University of Maryland, had to detour around the Statue of Liberty, but still finished in ninth place after 8 hours and 40 minutes.

Art Smith was seventh. The toughest part of his swim was keeping his mind occupied, he said. For that purpose he "sang '99 Bottles of Beer on the Wall' into the ground."

Asked what he did to celebrate his finish, Smith said, "I took one of the world's greatest hot showers."