Kimberly Neander spent yesterday morning in her allergist's office in Hyattsville, hoping for relief.

For Kimberly and 35 million allergy and asthma sufferers like her, shimmering warm weather has its drawbacks as well as its pleasures. For them, 'tis the season to be sneezin'.

"I don't sneeze just once," said Kimberly, 13, of Frederick, Md. "I sneeze 20 times. My nose itches. Sometimes I get hives. Sometimes, my eyes are swollen shut."

For the more fortunate, yesterday's weather, though a trifle sticky at times, was an automatic signal to go outside and play. They were gliding down the Potomac River, or leaping the rocks at Great Falls, or listening to fifes on the Alexandria waterfront.

Yesterday's high was 89 degrees, 5 degrees shy of the record for the date set in 1941, according to the National Weather Service at National Airport.

A forecaster said the temperatures were about 20 degrees above normal for a late-April day because of a high-pressure system off the mid-Atlantic that is circulating hot air into the region.

Several thousand people sweltered the day away at Alexandria's sixth annual Seaport Festival.

Tourist buses draped with the word "Smile" lined the streets around City Hall, and those parade perennials -- the Budweiser Clydesdales -- clopped their way through Old Town as Roy Orbison's "Pretty Woman" blared in the noon heat.

"It's so quaint, I feel like I'm in history or something," said Ellen Jamison of Nashville as she gazed at the shady elegance of Prince Street. "You really get the feeling this is the way the country was."

As always in Alexandria, history was well represented at the festival. Colonial balladeers crooned and dancers twirled to Scottish reels as large clusters of people watched.

The 3rd U.S. Infantry Army Band, known as "The Old Guard," marched along the waterfront in bright red coats and tri-cornered hats as dozens of people riding down the busy towpath stopped to watch them beat their drums.

"Is this an American tradition?" asked Jumichi Nishijima, a Japanese doctor who was earnestly filming the Clydesdales from every possible angle with his video camera. "We have to show them to our friends at home."

As the temperature soared, it clearly became a day for dark sunglasses.

It was also a day to get wet, and thousands of people from Mount Vernon to Great Falls plunged into the cool Potomac.

"Today is a day when you have to watch it, because many people here are so excited about this summer weather that they forget where they are swimming," said a spokesman for the U.S. Park Service at Great Falls.

Nobody drowned yesterday afternoon, but park rangers were on alert at the Little Falls Dam in Maryland, where last year 14 persons drowned in the stretch between River Bend Park and Key Bridge. In the only reported accident, a U.S. Park Police helicopter rescued five men after their boat capsized north of Great Falls yesterday afternoon.

Bright signs warned swimmers away from the deceptively deep waters near the dam. And colorful buoys bounced in the brown water.

Not everyone was deterred, however.

"We're perfectly okay," said Kevin Spungeon of Rockville as he and his brother Charles skipped across the rocks south of Great Falls. "We do this all the time, and we are very careful."

Sooner or later, however, many of the revelers found their way to drugstores in search of antihistamines and decongestants to relieve sniffly noses and weepy eyes.

"We can't keep them in the store," said Dorothy Hunter, a clerk at Medco in Old Town. "We're probably selling about a pill a person."

Several area physicians say that the 1985 hay fever season is no better or worse than in previous years. They are not seeing an alarming number of new cases, they say, only a steady stream of uncomfortable veterans.

On Friday, the D.C. Lung Association reported that the pollen count in Washington had reached 196, "the unhealthy range."

"The problem is, everything is pollinating right now," said Dr. Emine Cay, who treats allergies in Fairfax. "The trees are pollinating. The flowers have decided to pollinate. Although it's beautiful, it's also very aggravating and inconvenient."

Toward the end of May, the pollen season for trees and flowers will be over, said Dr. M.H. Amirgholi of Oxon Hill. By then, allergy sufferers will have something new to dread.

"Next," Amirgholi said, "come the allergies to grasses and weeds."