'THERE'S NOTHING to do at Fenwick Island," a friend told me. She is an old hand at vacationing in this antique Delaware beach cottage town, and said it with satisfaction, with the sort of quiet pride people take in a great-great uncle who was hanged for a horse thief.

And of course she was right, for there is nothing at Fenwick Island: no night clubs or discos, no amusement park or arcade, no boardwalk. Nothing, in plain fact, except sand, sky and immemorial sea. At Fenwick Island you can lie on the beach, sit on the porch or fish -- naive enough amusements, and those whose days they so fully fill obviously a dull lot.

Fenwick Island is typical, perhaps, of the tiny no-frills beach towns that once dotted the Atlantic seacoast from Florida to Cape Cod, and that are rapidly submerging into sleek chrome-plated anonymity beneath the relentless tide of real estate speculation, before the unpitying march of commercial progress.

There are still, certainly, many other pockets of repose along the broad Atlantic shores: Cape Cod, Nantucket and Martha's Vineyard, parts of Long Island, the Outer Banks, even corners of Florida and New Jersey. But they are all threatened, some closely, some more distantly (deceptively so, perhaps) by the specter of Miami, Virginia Beach, Ocean City and Atlantic City, by the accelerating encroachment of seaside urban sprawl.

Fenwick Island is an excellent spot from which to observe this phenomenon at close remove; its southern limit is the "Maryland Welcomes You" sign, beyond which a half-mile or so loom the high-rise condominiums of Ocean City, rearing up in vertical defiance of the ancient rhythms of sea and shore.

We have met the enemy, as Pogo used to say, and he is us. Twelve years ago I stayed one summer at 69th Street in Ocean City, in a simple frame bungalow more remote then than my cottage at Fenwick Island is now. It was a time and place for bonfires on the beach at night, for sitting out late in rapt worship of the stars; and it was, even then, a light year removed from the hustle and bustle of downtown O.C., where the night was made not for contemplation but ceaseless wandering.

Forty or 50 blocks south, among the all-night amusement arcades and hot-dog stands, one could not even sit on a boardwalk bench at night for longer than a few minutes. Even at 3 in the morning, brusque cops, fired no doubt by a restive Chamber of Commerce's alarm at the spectacle of vacationers sitting silent and unspending, would quickly shoo the immobile back into the nervous ebb and flow of commercial resort life.

"And by this black beach," John Galsworthy once wrote, "man is collected in his hundreds, trying with all his might to take his holiday. Here he has built a theater within the theater of the night . . . and round about lit lights to show him as many as may be of himself, and nothing of the encircling dark . . . and put a band, armed with pipes of noise, to drown the troubling murmur" of the sea.

Change Galsworthy's "hundreds" to "hundred thousands" and his "band" to "boom box" and the words still ring true -- truer now than when they were written. Sixty-ninth Street is only a memory now, no longer a place but an address, the condos risen in monumental vacuity for miles along the beach front, as drearily empty most of the year as new family mausoleums.

Even Bethany Beach, once nearly as sleepy as Fenwick Island, boasts a quasi-urban skyline of its own; while once-genteel Rehoboth, the self proclaimed "nation's summer capital," has acquired a reputation as a singles stalking ground and "Fire Island of the South."

Enter (or rather, remain) Fenwick Island. An anachronism: its beaches unfenced and uncrowded, its stars undimmed by more than a few pallid street-lamps, its dunes intact and alive with untidy beach weeds ("Our dunes are important to us" admonishes a mimeographed sheet of town rules and regulations.), the triumphant roar of the ocean drowning into insignificance the occasional portable radio.

It's a dull little town and one of a vanishing breed. For just as the metropolises of the east are merging, it is said, into one indistinguishable Boswash, the Atlantic beaches could soon, in the absence of sound planning and conservation, coalescue into a littering Floricod. It is little more than a generation now since Florida was a gravel-road fastness, and in less than another, the Atlantic Ocean could become little more than the front yard of Vacationland, U.S.A.

We don't need still more renovation for our dwindling seashore heritage, but preservation -- it is high time to stop selling and rezoning a piece of the rock to every developer with a deep pocket and a glad hand. There are Ocean Cities and Virginia Beaches enough now to satisfy the throngs who go to the seaside to watch one another; but there are unlikely to be any new Fenwick Islands to nourish those misanthropic few who simply wish to go down to the sea in peace.