IF IT WEREN'T for a mapmaker's mistake back in Lord Baltimore's day, all the land south of Lewes, Del., might have been bequeathed to Maryland - and Ocean City's string of condominiums might extend uninterrupted another 20 miles north to the mouth of the Delaware Bay.

Instead, the Delaware coast remains largely undeveloped - and overlooked.

To be sure, there's Rehoboth Beach with its mile-long boardwalk and Bethany with its Prohibition-era laws. But much of the Delaware coast line, 18 miles to be exact, is preserved and untouched, a part of the state park system since 1967.

For $4 a day, or $30 a season, a carload of out-of-state visitors has access to beaches and breakers, fishing piers and dunes that are never more than one-tenth occupied.

While you may not like paying for your suntan, actually the state beaches are a bargain. There's little commercialism there and no sales tax to drain your resources. In fact, the average daytime tourist spends only $6 a day on the Delaware coast, according to the state travel office, and the entrance fee keeps the crowds away.

You can sunbathe all day at the state parks and never suffer from sand kicked in your eye or an umbrella poked in your ribs. You can gaze at the sky without seeing a high rise - just a blue heron on the horizon.

At Lewes (pronounced Lew-is), where the ocean meets the Delaware Bay, the 2,500-acre Cape Henlopen State Park lures beachcombers to explore the wreckage of one of last summer's tall ships run aground, as well as World War II artillery range towers.

On Delaware Bay, the boisterious Atlantic breakers calm to gently lapping waves. Even though vintage clapboard homes stand near the bay shoreline the beach here is free and public - yours for wading, shell hunting, surf fishing or beaching your boat without fear of vandalism.

Because it's a sleepy fishing village still reflecting on its historical prime, Lewes is an escape from the regulations (and sometimes the rabble) of the popular beach resorts. Life slows to the point where you can enjoy an evening listening to blue jays and orioles from your front porch, or join your neighbors sitting at small bonfires on the beach.

Lewes has five motels inside the city limits and more than 100 cottages and waterfront townhouses renting for from $110 to $300 a week. Down the road a half mile at Henlopen State Park, there are camping facilities with a choice of primitive tent sites or more developed areas with access to hot showers and dumping stations. Youth groups may rent out an old Army barracks, now converted into a camping site with well-drawn water.

An advantage of camping in the park is ready access to the hiking trail through the Gordon Pond Wildlife Preserve. Here, in delicately balanced brackish ponds, the walker might see blue heron, Canadian geese or deer along the edges of the pine woods.

But the park's unique feature is an area of sandy hills called the "walking dunes." These dunes have departed from the beach and lead their own life, moving inland at the rate of 5 to 14 feet annually. According to Bill Beauchamp, Lewes' unofficial historian, a cranberry bog he visited in the dunes area last fall has been covered already by a plateau of sand 50 feet square.

National Geographic has described the dunes as a "miniature Sahara." Indeed, to walk through the sand is to view skeletons of tree tops, marram grass and such desert plants as prickly pear cactus and heron bill. Even though they seem inhospitable to animals, the dunes are the habitat of toads, non-poisonous snakes, squirrels and insects like the ant lion.

Twenty years ago, trucks drove right into the dunes to haul away tons of sand for factories manufacturing glassware or household cleanser. This practice has come to a halt as Delaware has grown ecology conscious. In fact, no longer are any vehicles, not even dune buggies, allowed on the "walking dunes."

Historical factors have also helped keep not only the Lewes area but also the entire Delaware coastline from overdevelopment.

During the World War II, the military turned the Atlantic shore south of Lewes into a key coastal defense. There were no invasions, of course, but espionage must have occurred because a sunken German U-boat contained bread wrappers from Delaware grocery stores.

When the federal government turned over most of the seacoast area 10 years ago, Delaware took advantage of the existing roads and buildings by creating public parkland. Tradition dictated that step. After all, back in the 1600s, William Penn himself had inaugurated a policy of public ownership of beach property by granting large stretches along Delaware Bay to Lewes' poor. Today that land can still obly be leased, not bought.

Elsewhere on the Delaware coast, a number of resort communities, including Rehoboth and Bethany, were founded by religious groups. They valued the beach more as a spiritual refuge than an economic resource, and fostered the notion in Delaware that waterfront property belongs to the people.

"All we have down there is water," lamented an official in Dover who argues that Sussex County, which encompasses the state's beaches, should develop its cities and shoreline enough to capture more tourist dollars.

"The provincialism of Delaware is what has kept that area from being developed," said another official who asked not to be identified. "Why not construct a Sea Land or a dolphin show? Quite frankly, it would get the people to spend more money. We should develop the area to a point where it doesn't do any harm to the environment but does become a good economic stimulus."

State Travel Officer Donald Mathewson, who remains neutral in the dispute says a recent survey showed that the average daytime visitor from Washington and Baltimore spends $6 in Delaware.

"They use 70 miles of Delaware highway and state services and the state receives very little of this at all," Mathewson said.

"The area is not utilized to best advantage," he added. "On some land, the average use is four duck hunters per acre in hunting season."

However, townspeople like motel owner Bill Beauchamp wanted changes to be gradual and carefully considered. Although part of Beauchamp's livelihood depends on the tourist traffic, he's glad local authorities are stingy with building permits. Lewes is recognized as "the first town in the first state" (the oldest settlement in the first state to ratify the U.S. Constitution), and residents want the town's appearance to reflect this.

Touring the historic homes of Lewes makes for a pleasant rainy day activity, particularly the group of six restored buildings painstakingly moved to Lewes from cities and backwoods areas all over Sussex County.

Two large restored homes, as well as an authentic general store, a spring house and an old fashioned outhoused are open to the public. But what really sparks the imagination is the log cabin - probably a slave house - with a double bunk, a stool, a hearth and a home-made ladder leading to the top bunk bed that's padded with an old patchwork quilt.

There are other preservation efforts in Lewes: the Zwaanendael Museum commemorating the original Dutch settlement in 1631; cannonballs from the War of 1812; homes with foundations made of bricks brought from Holland as ship balast, and the Overfalls Lightship, a floating marine museum.

However, Lewes isn't entirely set in past centuries. Conveniences are nearby when you feel the need. You can jog to the general store for the morning paper - available from Philadelphia, New York and Washington. By car you're just 15 minutes from Rehoboth's carousel and auction gallery, the supermarket on the highway or the twin bill at the Midway Palace I and II.

And Delaware residents have discovered that the New England coast doesn't have a monopoly on lobster. Most families renting at Lewes fine they can afford such a treat - not in a restaurant, but at home, for the rented cottages usually come equipped with lobster-sized kettles.

For dessert, it's hard to beat the sweetness of those Eastern Shore cantaloupes and peaches on sale at roadside stands. (A favorite is the Farmer Girl Market, an all-female enterprise.)

Or take the family for homemade ice cream at King's grocery store a few miles inland at Milton, Del. Here Earl King cranks out cones, pints and gallons in 17 flavors.

Fishermen don't need to be told about Lewes. They already know that fishing in the area is excellent, especially for sea trout. Charter boats costing about $125 a day leave the town dock to fish not only for trout but also for mackerel, croakers, hardhead, codfish, whiting and winter flounder - all in season, of course.

Headboats are available for about $12 a day. For around $7 neophytes can test their sea legs on half-day trips on the bay, renting rods and reels and even receiving instructions. Naturally, the free fishing piers should not be overlooked.

Another kind of excursion is available on the Cape May-Lewes Ferry. This ferryboat makes the 11-mile run to the South Jersey shore in 70 minutes, six times a day in summer, more often when the crowds demand it.

You can buy a round-trip pedestrian ticket for $3.50, an exceptional rainy day ace-in-the-hole. Or for $8 one-way, plus $2 for each passenger, you can take your car and explore the New Jersey resorts for the day, then slip back to the calm of Lewes at nightfall.