This year's extraordinarily wet weather may seem a damper on beach-going, but it will be a boon for sand- seekers who hate the traffic on the William Preston Lane Jr. Memorial Bridge and the distance to the Atlantic shores. The sea nettles in the Chesapeake Bay, and particularly in its tributaries such as the Potomac, should be fewer and later arriving.

And, unless there's an abrupt turnabout by Mother Nature, such bathing havens as Colonial Beach on the Poto mac and Sandy Point State Park where the Severn meets the Bay could be nettleless all summer. It's been years since that happened.

The dread stingers need a minimum of about 15 parts salt to 1,000 parts water to thrive, say the folks at the Chesapeake Bay Foundation, and right now that ratio is found only in the lower Bay. A recent sampling at Point Lookout, Maryland's southernmost point on the western shore of the Bay, showed salinity at just below that ratio. Imagine the possibility of venturing off Point Lookout's sand into the water with impunity after July 4!

Of course, the beaches right on the western shore of the Bay, from Highland just south of Annapolis to Calvert and Long Beach in Calvert County, will eventually have nettles, in increasing density to the south. But carefree swimming is the prospect until the dog days are nigh.

Antediluvians who date back to B.B.B. times (Before Bay Bridge) remember that the summer exodus from Washington used to be to the Potomac or Bay beaches, and you could tell the beachers -- they were the ones who put in for vacations in early summer, before the nettle invasion. The string of dozens of sandy stretches absorbed the migration, but it was crowded. Nowadays you can spread a blanket and have open sand between you and the next one.

Of course William Preston Lane Jr.'s memorial wasn't entirely to blame for the desertion of the Bay and river sands. More or less coincidentally came the outlawing of slot machines, and the pavilions at places like Colonial Beach, Mayo, Beverly and Dares stayed shuttered all year long. Most of them now are gone; a few are having a mini- revival as video arcades. Mostly, now, the beaches are just beaches. And this year, you can swim at them even when it gets warmer. BEACH HOPPING ON THE BAY SANDY POINT -- Take U.S 50 toward the Bay Bridge, exiting just before (there are signs). SAND SOUTH -- Or, exit on Route 2 south, just before Annapolis, and practically every road east takes you to a beach, public or semi- public. ON THE POTOMAC MARYLAND -- On the north bank, the river has so many feeders that there's no highway running parallel; you have to consult a map and head for a specific beach. For instance: Cobb Island -- Take U.S. 301 south to Route 257 to Route 254; Colton's Point -- Take 301 to 234, east to 242 south; Piney Point & St. George Island -- Take 301 to Route 5 to 249 south; Point Lookout -- Route 5 until it dead-ends. VIRGINIA -- Take 301 across the toll bridge at Dahlgren. For Fairview Beach, go west on 218 to 696; for Potomac & Colonial Beaches, take 218 east to 205 east; for Coles Point & Sandy Point, continue on 301 to 3, east to 612 or 604.