DUBOSE HEYWARD, who wrote the novel on which George Gershwin's "Porgy and Bess" was based, once observed, "it is always Sunday on sea islands.

Heyward may well have been reflecting on the area around Charleston. S.C., the Low Country Sea Islands in which Porgy and Bess played on their lives. There in a curving line from Charleston to Savannah, Ga., the islands are separated by lazy coiling rivers and creeks, edged by wide beaches of firm white sand, forested with palmettos, pines and live oaks that are decorated with lacy trailings of Spanish moss.

The vistas are placid and expansive; flat marshes fringing islands, shirimp boats festooned with a web of nets and gliding smoothly through the deep blue water.

Here in many of the marshes and beaches of the South Carolina Low Country, time moves slowly, much like a Sunday afternoon stroll -- which Heyward may have had in mind when he made his observation.

But civilization and its Monday morning mood of bustle and busyness has come to the Sea Islands, too. In the last two decades -- ever since a modest seaside motel named the William Hilton Inn opened in 1959 on the then-islolated island of Hilton Head -- resorts have blossomed in the area like daffodils in March.

One of the first Hilton Head's Sea Pines, in the early '60s, with its conscientious eye toward preserving the calm, primitive feel of the sea islands -- although, in fact, many marshes were filled in and smoothed over as the resort was built. Sea Pines developers preserved or created forests, lagoons, bird sanctuaries. Villas -- the resort version of condominiums -- and private clubs were constructed of faded cypress, in passive hues of light brown and green and gray, materials designed to blend in with surrounding nature.

Development spread to other islands -- including Fripp, Kiawah, Seabrook and the Isle of Palms. What once had been the province only of the animals and trees and creeks and poor people trying to eke out a living from nature has become the playground of wealthy and, increasingly, middle-class humans.

Vacationers, career-changers and retirees flock to the Sea Islands for the region's soothing semitropical ambience golf, tennis and water sports. They also come for the area's closeness to northeastern and midwestern urban centers. Why go all the way to Florida or the Bahamas, many ask when you can have the sun, sand, surf and sport so much nearer? Indeed, Hilton Head itself lures more visitors each year than Bermuda or Jamaica and almost as many as the Bahamas -- and estimated 800,000 in 1980.

The Sea Islands, now so popular with modern vacationers, were originally inhabited by the Yemassee and Kiawah Indians and were explored by the Spanish in the 1520s. In 1562, a band of French Huguenots led by Jean Ribaut founded a temporary settlement in 1562 on what is now Paris Island, the U.S. Marine Corps training depot. English sea captain William Hilton explored what is now Hilton Head (named for Wiliam, not the later hotel magnate Conrad) in 1663.

A distinctive form of architecture, featuring wide verandas and a central hallway to catch sea breezes, emerged on the plantations and in the towns. Builders developed tabby, a durable mixture of oyster shells and lime, to use in construction. Today, the cities of Charleston and Buefort are showcases of antebellum architecture.

The first shot of the Civil War was fired by Confederates toward union-held Fort Sumter, near Charleston, in April 1861. Soon after, a union fleet captured the town of Beaufort and the islands nearby -- including Hilton Head -- and established a crucial base from which to block Southern ports, thus severing the Confedersates' line of survival. Slaves got their freedom.

In August 1893, thousands died as a hurricane flodded the islands. A journalist named Joel Chandler Harris came to report on the devastation and stayed to collect from island residents the beloved "Uncle Remus" stories. Since September 1959, when Hurricane Gracie tossed her furious 120 mile-per-hour winds over the area, no major storm has erupted. Thus, none of the resorts has had to whether a severe hurricane, and many wonder if they would be able to. Some meteorologists say another big hurricane is overdue.

In additon, oil companies have drilled for oil off the South Carolina coast. So far none has been found, but tests are expected to continue.

The potential for destruction from hurricanes and pollution from oil drilling and refining poses a threat to the Sea Island resort industry. But, as tourist and sales business only gets better and better, such possibilities seem remote and unreal.

Most resorts surveyed reported increases in business for the last year and in bookings for the early months of 1981. Marketing and sales managers said that, even with the current recession, people are willing to spend -- for a motel room, a rented villa or private home or share in a condominium under an arrangement called "time sharing" -- if they get a serene, uncluttered, relativley uncommercial atmosphere in return.

There is some attempt to introduce Sea Island culture, primarily in cuisine -- noted for she-crab soup (made with crab roe, cream and sherry) and red rice (rice cooked with salt pork and tomatoes) -- and in such crafts as baskets woven of honey-colored sawgrass. Some of the resorts are called "plantations although no crops are grown on them. The indigenous culture often seems injected by people who do not really care about it, and thus loses its heart and soul, coming across as artificial overlay on resort life.

At first glance, the resorts seem almost stereotyped in their similarity: buildings of faded cypress, profusions of golf courses and tennis courts and bicycles and small expensive shops, tennis clinics and children's programs and oyster roasts, well-scrubbed and consciously healthy people walking around in dazzling primary colors they would not dare wear to the office.

All the resorts offer tight security, as evidence by formidable gates equipped with unsmiling uniformed guards who briskly turn outsiders away.

But there are differences, however subtle, that may make some resorts more suitable than others for your needs. If you are planning a trip to the Sea Islands, you might well look into these differences before making your reservations. Hilton Head

This 28,000-acre boot-shaped island boasts 13 miles of beach, 11 golf courses, more than 125 tennis courts, six marinas, 10 shopping centers and more than 600 shops and businesses, including more than 100 restaurants. It has a modern library with 14,000 books, a hospital, a private airport and a four-lane highway running through the center. (Hilton Head includes the Sea Pines, Palmetto Dunes, Shipyard Plantation and Port Royal Plantation resort areas.)

Hilton Head's permanent population numbers 10,700. The Island Packet, the local newspaper, publishes complaints that the island has become too crowded for its own good, and some visitors bristle at being asked by resorts to schedule their golf and tennis sessions at the time that they make a reservation. Others find they cannot reserve the playing times they prefer.

In short, Hilton Head is bustling. If you think you need a varied supply of shops and movies and nightlife, one of Hilton Head's resorts may be what you're looking for. You can still retreat behind the security gate to the peace and calm the "plantation" purveys.

To get to Hilton Head and its resorts by car, take I-95 south from Charleston and exit onto Rte. 462, which becomes Rte. 278 the island's major highway. By air, Eastern, Delta and Pan Am fly into the Savannah airport, from which the resorts provide limosine service. Amtrak has regular service to Savannah. It's a 45-minute drive to Hilton Head from Savannah. Sea Pines

Philip Lader, the gregarious president of Sea Pines, reports happily that the resort did more business than any other in the country last year -- $25 million in visitor revenues. Sea Pines has three golf courses (fees are $20-$25 for 18 holes), 72 tennis courts ($12 an hour), 15 miles of bike trails, more than a dozen swimming pools, a 572-acre forest and wildlife preserve, a charter fishing fleet, riding stables and about five miles of beachfront. To offset all that outdoor activity, there are 15 restaurants and lounges.

The focal point is Harbour Town with its red-and-white "lighthouse" (it is fake) and its promenade of expensive shops and restaurants, lined with rocking chairs for people-watching and yacht-gazing. The Harbour Town yacht basin can accommodate 90 boats. Besides the Heritage Golf Classic in March. Harbour Town hosts the Family Circle Cup tennis tournament in April.

Guests may stay at the 200-room Hilton Head Inn (formerly the William Hilton Inn), in one of 900 villas with from one to four bedrooms or in one of 125 private homes with three to six bedrooms. Rates through March 20 at the Inn are about $73 daily. After March 20, the range is $90-$100 daily. A two-bedroom villa will cost $80 daily through March 20. A three-bedroom house is about $60 daily through March 20.

For reservations, write Sea Pines Resort, Hilton Head Island, S.C. 29928 or call toll-free 800-845-6131. Palmetto Dunes

This 2,000-acre resort opened in 1971 and is anchored by the huge, hulking white Hyatt Hotel. There are three restaurants, two golf courses (fees are $16 per 18 holes), 25 tennis courts (at $13 an hour), 18 swimming pools and rental canoes a visitor may use to explore the seven-mile network of intersecting lagoons.

Rates at the Hyatt from February to November are $100 on the "sunset side" (which means the non-beach side) and $120 on the ocean side. Rental costs for one of the 450 villas, which have from one to four bedrooms, range from $50-$160 daily, with $70-$85 daily the average fee for a two-bedroom villa.

The resort is designed for walkers, as the villas are within walking distance of the beach, the pro shops, the tennis courts and the Hyatt, and swimming pools are scattered everywhere. The resort hopes to open a marina at the end of 1981 and add 100 villas and a 340-room hotel by 1982.

For reservations, write Palmetto Dunes, P.O. Box 5606, Hilton Head Island, S.C., 29928 or call 800-845-6130. Shipyard Plantation

Shipyard is smaller than Sea Pines and Palmetto Dunes, with less than one mile of beachfront (although visitors may then range anywhere they choose on Hilton Head's beach), no restaurant and no hotel, although Marriott Hotels plans to open a 341-room inn there by May 1, charging $70-$90 daily through November, with two restaurants and a lounge. There is no children's program at Shipyard now, but Marriott may begin one.

Visitors may stay in one of about 150 villas of from one to three bedrooms, with fees ranging from $80-$130 daily starting in March. The villas may be as much as a 15-minute walk from the beach, but many include fireplaces and skylights and some include free tennis on private courts.

There is one golf course ($14 for 18 holes) and the Hilton Head Racquet Club has 20 courts ($10 an hour). Nine more holes of golf should open by October.

For reservations, write Shipyard Plantation, P.O. Box 1786, Hilton Head Island, S.C. 29925 or call 800-845-6135. Port Royal Plantation

The Port Royal Inn has 66 rooms, charging $66 daily in March for an oceanfront room. The Inn also has one- and two-bedroom suites ranging from about $85-$175 daily. About a dozen private homes of three to four bedrooms are available to rent, with fees from March to October ranging from $850-$1,100 weekly.

This is a low-key resort, with no children's program and no condominiums. There are two golf courses ($14 per 18 holes), four free tennis courts, one restaurant at the Inn, fishing, biking and two miles of beachfront.

For reservations, write Port Royal Plantation, P.O. Box 1786, Hilton Head Island, S.C. 29925 or call 800-845-6135. Fripp Island

Fripp, with 3,000 acres and 3 1/2 miles of beach, offers isolation and less of what some would call the pretentiousness and others would call the elegance of the Hilton Head resorts. More than any of the other resorts, Fripp feels less consciously planned. The Inn, though large and modern, has only about a dozen rooms available to rent (the rest are used as conference rooms) and charges about $50-$55 daily.

There are 175 villas with from one to four bedrooms, ranging in price from a two-bedroom "tree house" in the woods for $400 weekly to an oceanfront four-bedroom villa for $1,150 weekly. Most villas cost from $600-$700 weekly.

Restaurants may be found at the Inn and the marina, and there are a small grocery and deli at the entrance to the island and a grill at the pro shop. Otherwise, there is none of the business and bustle of Hilton Head.

The nearest city, Beaufort, with about 10,000 people, is 18 miles away.

Fripp includes one golf course ($14 for 18 holes) -- with an option to play also at the Royal Pines Country Club, about 15 miles away -- 14 tennis, courts ($12 an hour), a marina, charter boat fishing, bike and sailboat rentals and a children's program in summer. By spring, a Fripp executive said, more condominiums and another restaurant will be open.

For reservations, write Fripp Island resort, Fripp Island, S.C., 29920 or call (toll-free East of the Mississippi River) 800-845-4100.

To get there by car, take Rte. 21 south through Beaufort and follow signs to Fripp Island. Air and train service is through Savannah. Kiawah

This 10,000-acre island, owned by a group of Kuwaiti business executives, has 10 miles of beachfront. It is 21 miles from Charleston.

There are 150 rooms at the Inn, ranging from $45-$190 daily through March 12, and one- to three-bedroom villas costing $85-$180 daily or $425-$900 weekly through March 12. Four restaurants offer informal and formal dining.

Kiawah includes a mini-mall with gift, clothing and food stores. Outside, local women weave and sell Low Country straw baskets.

Activities include a golf course ($8-$10 an hour), bike rentals, swimming pools, fishing and crabbing. Shelling is especially good in the winter months, when severe storms wash larger shells up from the deep. In addition, special sailboats may be rented for "landsailing," or sailing on the sand, and a jeep safari takes guests on a 2 1/2-hour tour of the wild, undeveloped areas of the island. Special events include a running symposium Feb. 19-22 with speakers and races, and quarter-horse racing on the beach March 28.

For reservations, write Kiawah Island resort, Box 12910, Charleston, S.C. 29412 or call 800-845-2471.

By car from Charleston, take Rte. 17 south toward Savannah. Three miles south of Charleston a sign will direct you to turn left onto Bohicket Road, which takes you to the resort. Delta, Eastern, Pan Ann and Piedmont serve the Charleston airport, from which there is limousine service to Kiawah. Amtrak serves Charleston. Seabrook

"If you're a big splashy partyer, this isn't the place to go," says one Seabrook employe. The resort, 23 miles from Charleston, prides itself on being uncluttered. There are 3 1/2 miles of beachfront on the 2,100 acres. Although there are no motels, there are 280 units to rent in villas, cottages and beach houses with from one to three bedrooms. Villa pieces range from $50-$135 daily through March 15, then from $65-$175 daily through Nov. 15. Fees for private homes start at $500 weekly.

For activities, there are two beach clubs, three restaurants, a golf course ($18 per day) with another to open this spring, tennis courts ($12-$14 an hour), a swimming pool, horseback riding, sailboat and bike rentals, canoeing, fishing and crabbing.

For reservations, write Seabrook Island resort, P.O. Box 32099, Charleston, S.C. 29407 or call 800-845-5531.

To get there by car, take Rte. 17 south from Charleston to Rte. 700, which leads to Seabrook Island. The island is served by Charleston airport and Amtrak into Charleston. Isle of Palms

The Beach and Racquet Club at the Isle of Palms offers proximity to Charleston -- the historic city is 12 miles away. Its 1,500 acres includes 2 1/2 miles of beachfront.

Although there is no motel, there are about 140 rentals units of from one to three bedrooms in villas, cottages and private homes ranging, for a three-bedroom unit, from $95-$170 daily. One restaurant services the resort, in addition to a snack bar and a grocery store. Outside the security gate is the rest of the Isle of Palms -- including a commercial strip for those who need neon lights after the self-conscious tranquillity of the Beach and Racquet Club.

For visitors there is one golf course ($15 for 18 holes), 14 tennis courts ($8 an hour), swimming, biking, boat rentals at the marina, landsailing and, during the summer, sailboat rentals. Only half of the resort, which opened in 1977, is complete, and a great deal of construction is underway now.

For reservations, write the Beach and Racquet Club, Box Y, Isle of Palms, S.C. 29451 or call (803) 886-6000.

To get there by car, take I-95 to I-26 east, then Rte. 17 Alternate and follow the signs to the Beach and Racquet Club, which is at the end of the highway.