If Malcolm Forbes' yacht, The Highlander, had sprung a leak last night, half of establishment Washington would have been in the swim.

The 126-foot teak yacht did, of course, no such thing, but sailed down the Potomac in high style, with the Woodrow Wilson bridge going up in the air so the boat could go under, while guests dined on caviar, roast beef, oyster pudding and chocolate mousse. The evening sun turned the water to a molten gold, the air was neither too hot nor cold the company was august, the food the drink lavish.

For Forbes, publisher of Forbes magazine, who already has a world-class collection of Faberge eggs, presidential autographs, lead soldiers and toy boats, to name a few, it was a first-class collection of Washington power elite, circa Reagan.

Last night, some 60 guests were piped aboard the boat by bagpiper Jordan Forbes (no relation, he just turned up in a Forbes kilt with his bagpipes a year or so ago; now he's the steward on the 10-man crew).

Last night, ambassadors, tomorrow a prime minister. Zenko Suzuki, prime minister of Japan, will be guest of honor tomorrow at a luncheon of breast of chicken and lobster while The Highlander sails around Manhattan.

Former ambassador Walter Annenberg, protocol chief Lenore Annenberg, British Ambassador Sir Nicholas Henderson and Lady Henderson, and American ambassador-designate to Great Britain and Mrs. John Louis relaxed in the knowledge that their royal guest, Prince Charles, had a good time and was safely out of their responsibility. But for Sen. Claiborne Pell (D-R.I.) and Mrs. Pell, another crown prince's visit comes this week. Mrs. Pell said Prince Hans Adam of Lichtenstein will be their house guest.

Lady Henderson reported that Prince Charles enjoyed staying in the Laura Ashley bedroom at the British Embassy, done over by Lady Henderson in the British fabrics and wallpaper and with her own collection of Victorian music hall sheet music covers. But then Prince Charles, she said, is "so sensitive and simple to please."

Protocol chief Annenberg said the whole trip went very smoothly. "No problems," said said.

CBS commentator Roger Mudd congratulated Lyn Nofziger, assistant to the president for political affairs, on the job he had done keeping the country informed the day President Reagan was shot. "It didn't seem strange to me," Nofziger said. "I just did what I had to do." His wife smiled and said, "After all, he'd had 15 years in that job." He was Reagan's former press spokesman.

Malcolm Forbes, with two of his sons, Steve and Kip, gave tours of the boat to the fascinated guests. "It's our fourth," he said about the boat. "Our last one caught fire and burned completely. But we were lucky, a man had commissioned this one to be copied after our old Highlander, but larger. When the income tax law was changed, he couldn't afford to keep it, and we were able to buy it right away."

The boat sleeps 22. The luxurious grand salon is furnished with flowered cushions and glass-front cabinets full of Imari china plates and shell ornaments. On some walls are part of the Forbes autograph collection, including a thank-you picture from Queen Elizabeth and Prince Philip for the loan of a previous Highlander for transporting part of the royal entourage.

Roberta Forbes, Malcolm's wife, said this was the first time she'd been down the Potomac. "But then, I only come on the boat," she said, "when there's an event like this one."

The evening ended at 9:30 so the boat could make it to New York in time for the cruise with the Japanese prime minister, and the disembarking guests were handed souvenirs -- shopping bags reading "Forbes: Capitalist Toolbags," outfitted with the May issue of Forbes magazine, a box of Forbes Capitalist Cookies and Forbes' book, "The Sayings of Chairman Malcolm." Chinese Ambassador Chai Zemin and Madame Li Yufeng smiled as they took theirs.