AND NOW A special message for those who followed our mistaken advice last Saturday and showed up ready for the "Riverfest '85" celebration along the Maine Avenue waterfront: You were great to hang in there all week, and please don't give up, because your patience is about to be rewarded. We have it right this time: today and tomorrow the festival takes place. There will be tall ships, steel bands, dancers, ethnic and traditional outdoor foods and a fireworks finale tomorrow night. Jacques Cousteau is slated to show up too, for a welcome this morning from Mayor Barry, a fife and drum corps, the city's fire boat and, as they say, much, much more.

What was true last week and today as well is that the Potomac River is cleaner than it has been in years, thanks to the unusually fine cooperation of the governments in the region. The opportunities for safe recreation in this friendlier section of water have grown too, and that's what the event seeks to accentuate. For that matter, Washington's waterfront should be a popular spot year-round -- given some creative planning aimed at attracting residents as well as tourists, offices as well as dwellings and international organizations as well as small shops.

For now, Riverfest is on the immediate horizon. Really.