First there was the boat ride down the Potomac to Mount Vernon. Gin, vegetable dip, etc. Then limousines shuttled everyone from dock to front door. After that, the fife and drum corps played "Swanee River." And then, right after the shrimp dip and just before dinner on George Washington's old lawn, the sun set. On cue.

It was the American Stock Exchange's party for 400. It put even Reagan-age parties to shame.

Nobody appeared to know exactly how much it all cost, although one estimate from an Amex vice president put it at $200,000. But he amended that a short while later to an undisclosed figure described as significantly less.

"It adds up," said the vice president, Joshua Rich.

The party was the social highlight of the annual Washington meeting of the Amex, No. 2 compared to the bigger and more prestigious New York Stock Exchange. Amex companies are mostly medium-sized "growth" companies, often located in the Midwest. Amex executives say they don't have an inferiority complex, but then, listen to former Amex official George Reichhelm on the subject of the NYSE:

"They're asleep over there. They're fat. They're rich. They're complacent. And they're inbred."

Reichhelm was speaking as the Diplomat, a large party boat rented for the evening, motored pleasantly past the haze of National Airport and the swampy banks of the Potomac. The breeze lifted skirts.

On board were the executives of Amex companies, as well as Rep. Timothy E. Wirth (D-Col.).At the bow was Michael Tenzer of the Leisure Technology Corp., drink in hand and 3,000 miles from his Los Angeles office.

"Am I on top of the world?" he said. "I don't know. Interest rates."

It was that kind of crowd.

But even though the prime rate went back up again yesterday to 20 percent and even though Wall Street has criticized the administration's taxcut plan, the general view from this crowd was that Ronald Reagan's all right. For now.

"I think it's early to tell," said Amex chairman Arthur Levitt Jr., a cheerleader for small and medium-sized businesses. "But as I see the business community's thinking, I think they're favorable inclined toward the administration. We're not talking confrontational politics as much today as we did a year ago, two years ago."

And as for the administration's tax cut proposal versus the Democratic alternative?

"I think there are elements of both of them that are attractive," Levitt replied diplomatically. "The differences are miniscule."

Not too long after, the boat reached Mount Vernon. A long line of limousines waited, but for the adventuresome, there was a short hike through sweet-smelling foliage.

"George had a good eye for real estate," observed Chris Covington, a New York investment banker who was trudging along.

Pretty soon, he reached Mount Vernon, the site of only two previous parties: an official dinner given by First Lady Jacqueline Kennedy in 1961, and a party for French President Valery Giscard d'Estaing in 1979.

So just how did the Amex manage in 1981?

"You know, there's sort of an association that goes back a long time," said Frances Guy, regent of the Mount Vernon Ladies' Association that runs the landmark. "Back yonder, in about 1790 to 1792, they started a crubside exchange in New York. They helped George Washington those first few years of the presidency. It was something the country needed at the time, a central location for trading."

The curbside exchange eventually grew up to be the NYSE and Amex, which is the reason the Ladies' Association felt it only proper to help out somebody who'd helped George Washington. And as Frances Guy said after some prodding, somebody knew somebody in the Amex camp.

There was no charge for using Mount Vernon, but as a friend of the Ladies' Association explained sweetly: "We hope that everybody else here will be moved to donate."

Then dusk fell. The limousines patiently waited.