Certainly the purity of Potomac River water has improved these last few years, thanks in large part to concerted efforts to clean up wastes. But has it reached the point where, as depicted by the fanciful emblem seen to the right, Washington can rival Ocean City for catching what appears to be a huge game fish?

That's a quibble voiced in fun, which is what Potomac Riverfest '85 is supposed to be all about.

As the successor to a series of waterfront festivals, Riverfest seems destined to become a true local event for Washingtonians -- unlike the worthy, but chiefly tourist-oriented, Cherry Blossom Festival and the July 4 festivities that celebrate the city's role as the nation's capital.

Last year's Riverfest drew an estimated 100,000 visitors.

Mark your calendar for Riverfest '85 over the weekend of June 8 and 9, Mayor Marion Barry told a couple of hundred community leaders and officials who gathered yesterday for lunch in a transparent-sided tent on the Maine Avenue waterfront.

The event will include music, exhibits of arts, crafts and items of general interest and rides for the kids, capped by a fireworks display on Sunday night (when Metro will remain open till midnight).

Prior to and during Riverfest, 11 tall-masted sailing ships will visit the port, with the first -- the schooner Spirit of Massachusetts -- expected to arrive for a three-day visit at 11 a.m. Easter Sunday.

She'll be open to visitors from 11 a.m. to 4 p.m. Monday and Tuesday at the Gangplank Marina, Seventh and Water streets SW.

And, in conjunction with all this, Barry has chosen an admiral (and a vice admiral) of the Potomac. Adm. Clayton F. Cisar -- who, incidentally, already has been named an admiral of the Chesapeake by Maryland's governor -- was present at yesterday's event, resplendent in a white beard and an even whiter uniform. He's the charter president of the Potomac River Pilots Association.

The vice admiral, Howard Gasaway, commodore of the Seafarers Boat Club on the Anacostia River, couldn't be present.