Among this summer's surf-sand-sun set along the Delaware-Maryland are several hundred thousand Washingtonians, who make up the bulk of the summer population along the 30-mile stretch between Ocean City, Md., and Rehoboth, Del.

Seashore real estate has become increasingly attractive to people from this area bent on summer R & R or off-season escapes from paper clips, computers, contracts and customer services. This has been a boomer season at the beach, and visitor traffic is expected to be brisk during the autumn months as well.

The real estate market on the Atlantic shore 130 miles east of Washington is now make up of all kinds of investment and rental opportunites. In the off-season, you can rent a cottage or condominium or near the beach for about $75 a weekend. In the summer you can pay as much as $700 a week for a sprawling oceanfront house at Bethany Beach, Del.

You may also find it is impossible to get a hotel or motel room for a period of less than three nights: One Washington realtor wanted a Saturday night accomodation in mid-August and had to postpone a planned business trip because none was available.

But come Labor Day, the rental market changes. Real estate agents at the shore will tell you that the prime renting reason corresponds with the school vacation period.

"The final week in August (just prior to the still-popular Labor Day weekend) has lost some of its appeal here in Bethany, and probably eslewhere, because schools are starting up earlier than heretofore," said realtor Susan Connor, now a year-around resident of Bethany Beach, a relatively quiet settlement about five miles north of the Mason-Dixon line. Along the row of high-rise buildings that stretches north of Ocean City, Md., is the Carousel, shown here on the left behind its parking building. It was built on the site of the motel that former Senate.

Her view that his rental season was "excellent" was echoed up and down the beachfront by other agents. Most said sales of new dwellings and resales have also stepped up. However, other observers noted that the recent appreciation of resale prices has not kept pace with the dizzying spiral in and around Washington.

One bovious reason is that too many beachfront and bay-side condominium apartments were built in 1972 and 1973, particularly in norther Ocean City, and the market just now returning to to near normalcy. Remember, it was only two years ago that the glut of unsold condominiums at the shore exceeded 3,000 units.

A number of the new buildings were then taken back by lenders, who cut back prices and offered financing inducements to buyers. Some of the inducements were such that hundreds of new units were sold within a few months, notably at the Carousel, the apartment-hotel that was built at the site of the origainal Carousel motel and nightspot of former Senate aide Bobby Baker.

A decade after the Carousel was built, a building boom brought scores of high-rise apartments to aide Bobby Baker began in the early 1960s. Carousel condominium fees were recently raised from $80 to $160 a month. Marriott Corp. operates hotel rooms on the lower floors of the Carousel, northern Ocean City, an area that was regarded as "far out" when Baker pioneered there.

Traffic to the Atlantic Coast picked up considerably in this decade after the second span of the Bay Bridge was completed. Washingtons in increasing numbers have rented summer places and bought apartments, town houses and single-family houses oriented to the sea or nearby bays.

The large Ocean Pines recreation community was lauched by Boise Cascade just outside Ocean City and now is the site of hundreds of homes and even more lots owned by folks who one day plan to build or resell.

In the meantime, Boise has unloaded its lots there and is preparing to withdraw from the project.

As Ocean City development has moved northward five miles to merge with the tiny Fenwick Island, Del., community, the oceanfront had taken on a high-rise profile, changing the character of the once-isolated shore and turning off some visitors in the process - while turning on others interested in condominium living.

Some of the big new buildings have indoor and outdoor pools and the Carousel even has its own in- door ice skating rink, where you can see figure-eights being cut on a steamy Saturday afternoon in August.

While Ocean City, with its colorful boardwalk and honkytonk ambience, attracts much of the summer attention, there is increasing interest in its neighbor to the north, Fenwick Island, which has one high-rise and many houses and town houses, as well as attractive mainland waterway development. Fenwick's benchmark is its non-functioning lighthouse.

To date, there's been no real profit-taking by owners of high-rise condominium apartments along the shore, but some are expecting that market to improve just as soon as the inventory is exhausted. It is unlikely that many new projects will be started in the near future.

While home ownership has probably doubled at the beach since 1970, the ocean real estate market continues to be dominated by renters. Rents rose about 10 per cent this summer.

No one knows just how many of the new owners of condo apartments and town houses are renting them during the season to keep up payments. But some owners are said to be meeting the increased costs of association fees and utilities by taking in partners.

Rents and house prices at the beach look like near-bargains compared with real estate prices back in metropolitan Washington. But, like another waterside community to the West, Annapolis, the ocean towns are narrowing the price gap.

Next Week: What's happening in the quiet beach communities to the north.