Not a thing in the old mailbox yesterday? Don't blame your local Postal Service honchos -- they were out cruising the Potomac.

Cool breezes cleared away the beads of perspiration. Platters full of cheese, cold cuts and salad were picked clean and slowly returned to the lower deck. As the good ship "Diplomat" verred left at Mt. Vernon, the historic estate peeked through the greenery on the starboard.

"What we all have in common is mail," said John Murchake, subscription manager for Kiplinger Washington Editors and a member in good sailing of the Washington area's Postal Customers Council, which for the third year running decided to hold its annual meeting in waves.

Now don't get the idea that the cruise exists so irate Joe Letterwriter can develop sympathy for whoever crawled his last missive across town in two weeks. This was no trip for timmies who, as Van Seagraves, editor of Business Mailers Review put it, "think mail is anything a person able to lick a stamp can do." Commercial and governmental mailers, each responsible for large volumes of mail, paid $15 apiece to ask about things that bother the big-timers.

"I have learned more today about the new nine-digit zip than I would any other way," testified Dorothy S. Percival from the Army Corps of Engineers. Studying Postal Service leaflets at the various tables set up for questions about "2nd and 3rd Class" or "Express Mails," the scores of guests exchanged new methods and favorite memories of the Service -- ye olde revolving door between government and business spins just as swiftly for postal people.

They also discoursed on some fine points of mailing for an amateur.

"Never put tape over the whole stamp," warned Gwen Faggetter, customer service in representative for the Postal Service in Northern Virginia and a 15-year veteran. "A lot of people do it. It will be sent back because it can't be cancelled." Suppose part of your stamp breaks off as you tear it out of the book? "Use two-thirds," answered Murchake, "but don't ever use one-third -- it has to be 50 percent."

What about jokesters who scissor off the cancelled part of a stamp? "A lot of people do it," acknowledged Faggeter, adding that they were playing around with federal law. "The Postal Inspection Service has as much power as the FBI," she said with a touch of pride.

Once started, they dismantled myth after great American myth. Take the express letter, guaranteed to arrive tomorrow anywhere in the United States if deposited at a post office today. If it arrives late, tough luck, right? Wrong.

"You get your money back," explained Murchake, emphasizing that you'd better prove it.

Back in the late '60s, some people pasted on their stamps upside down to protest the war, among other things. Another misdeed that might bring the feds running ? "Not at all," said Faggetter. "In the service, that meant 'I love you.'"

As the "Diplomat" returned to port, even hardened postal veterans were shaking their heads at the story of an unfortunate playwright, desperate for a May 31st postmark, but caught in a post office line with June 1 just minutes away. He made it to the window at 12:07. The clerk refused him the May 31st postmark he needed for a contest deadline.

"There are always some guys like that," said one as he left the boat.