The Calypso, sparkling white and elegant, glided along the D.C. wharf yesterday morning and the crowds at "Potomac Riverfest '85" strained to see its famous master. A moment later Capt. Jacques Cousteau, lean and nimble in a slate-blue leisure suit, appeared on deck with a broad smile.

"Happy birthday, Jacques!" a woman called and the crowd began to sing. The oceanographer, television star and special guest of the waterfront festival, who was observing his 75th birthday, waved his hands elaborately as if to conduct the singers.

The purpose of Riverfest weekend, initiated last year, is to celebrate with food, water sports and music the cleanup of the once shamefully polluted Potomac River. It was clear yesterday that the prenoon crowd of about 5,000 was also celebrating the presence of Cousteau and his research vessel, the Calypso. D.C. recreation department officials, who supervised the event, said some of Cousteau's Washington friends invited him.

"He has the smarts. He has the charisma. He knows what he's talking about," said Gaithersburg homemaker Sharon Nichols, 33, a six-year member of the Cousteau Society who took a proprietary interest in Cousteau's appearance. "He has educated the world about the need to protect our oceans and rivers."

Cameras clicked and whirred. Small children sat on their parents' shoulders to get a better look. People crowded around the Cousteau Society stand to buy $20 annual memberships, Calypso T-shirts and model kits of the former World War II minesweeper. They watched enviously as Mayor Marion Barry Jr., the festival chairman, toured the Calypso, wearing a navy blazer and a white yachting cap adorned with gold braid.

The Calypso last visited the Potomac in 1959, two years after the U.S. Public Health Service declared the heavily polluted river unsafe for swimming and more than a decade before a $600 million campaign was launched to reclaim the waterway.

"In 1959, I don't think the river was in the same state as it is now," Cousteau said yesterday.

The Potomac, which begins as a small spring in Backbone Mountain, W.Va., and continues for more than 300 miles before emptying into the Chesapeake Bay at Point Lookout, once teemed with wildlife.

By 1969 it was "grossly polluted and a severe threat to the health of anyone coming in contact with it," according to a Potomac Enforcement Conference meeting at the time. A cleanup was launched in the early '70s, and by 1976 pleasure boats and large-mouthed bass had reappeared in the river.

While no one claims the Potomac is pristine now, it is vastly improved. "You can fish in it," Barry said yesterday before presenting Cousteau with a plaque and a District of Columbia flag, "but more importantly, you can eat the fish that you catch."

Organizers predicted that 200,000 people would attend the two-day event, which ends at 10 p.m. tonight with a fireworks display. Last year, about 100,000 attended the festival.

Children gathered around the aquarium set up by the Maryland Department of Natural Resources, pressing their fingers on the glass as if to touch the rainbow trout, black crappies and walleyes.

Outside Grover Barbecue's steaming stand, about 50 people stood in line for ribs and chopped barbecue. Few were attracted to the nearby table displaying "Nonviolent Foods," a vegetarian repast.

To some, it was a little anticlimatic after Cousteau. Lisa Lewis of Temple Hills ran up to her husband, Robert, a Federal Reserve employe, and buried her nose in his shoulder, exclaiming, "I shook his hand!"

"We really believe in Cousteau," said Robert Lewis. "That's why we came."