In contrast to the hard times in residential real estate inland, construction, sales and rentals at seaside resorts are on the rise from Ocean City, Md., to Rehoboth Beach, Del. At the same time, what is considered the peak season at the ocean is expanding. July and August still are the key months, but the number of visitors in May, June, September and October is increasing steadily. Many beach-goers regard late September and early October as the best time by the sea since days are sunny, evenings cool and beaches less crowded.November, February and March still are slow but only December and January now are considered true "off" months.

In the first fourt months of 1981, Ocean City issued permits for $15.5 million worth of construction, compared with $5.4 million during the same period last year. Among projects currently planned there are a total of 375 condominium units divided among several different buildings and a 120-unit Holiday Inn at 66th Street.

In a recent report on the situation in Ocean City, real estate consultants Lipman, Frizzel & Mitchell of Baltimore and Silver Spring credit the longer seaside season for an overall increase in construction permits and sales tax receipts in 1980, when real estate sales volume rose 14 percent compared with a 9 percent decrease in central Maryland. Construction of both hotel and condominium dwelling units in Ocean City also went up last year. Condominium appreciation there is generally more than 10 percent annually.

The Ocean City report, which includes interviews with real estate agents, notes a tendency for buyers to pick lower floors of high-rise buildings, apparently out of concern about fire dangers or because of "some rather long waits to get elevator service during the height of the season." This suggests a trend diametrically opposed to that of earlier years, when buyers paid more to be on higher floors.

The report indicates that less-expensive, low-rise bayside development also has picked up, possibly because of "intense population density on the ocean side of the Coastal Highway during the peak season." In addition, traffic congestion on the Coastal Highway is regarded as a "nagging problem" from 6 to 8 p.m. during the peak season. Ocean City Mayor Harry Kelley said that the city has purchased new buses and that 30 of them will be available to transport vacationers this summer.

Kelley, who has been mayor of Ocean City for 11 years but has confirmed that he seriously is planning to run for the Democratic nomination for governor of Maryland, ascribes Ocean's City's lure to its family image, strong but pleasant security and good beachfront. The mayor, who recently sold a house there to move into a bayside condominium apartment, said the city is taking steps to preserve the beach front by constructing six more stone jetties built with 5-to-7-ton stones to a height of 8 feet, 250 feet into the sea from the beach. Kelley also said the city has completed a small park near the entrance bridge on Rte. 50.

Real estate broker Bruce Moore, a veteran of 13 years in the Ocean City market, said that the present boom at Ocean City is different than the one followed by a bust in the mid-1970s. That period of foreclosures and bargain-price sales was the result of over-building, speculation, a mortgage crunch and a gasoline shortage.

Moore said that today's buyers at the beach accept rising prices because they have investment incentives but are not interested in cash return. He stressed that buyers are often young, affluent couples with two jobs. "There's also an ego factor in ownership of a place at the beach," he said.

Not suprisingly, those who own property in Ocean City include a number of prominent Washington real estate professionals. In addition, Moore said, sales are up because more Washingtonians want to vacation near home. But he and other brokers have noticed increasing numbers of summer tenants coming from cities farther away such as Pittsburgh.

Chesapeake Bay Bridge officials can confirm that there is increased beach-going from Washington and Batlimore. In 1975, bay crossings in passenger cars totaled 7.3 million. In 1980, the total was 9.3 million. Last year crossings topped 1 million in June, July and August.

Washingtonian Angelo Puglisi, who rebuilt Decatur House as a downtown condo in Ocean City, agreed with broker Moore about the boom in Ocean City, and noted that oceanfront development sites there costing from $9 to $13 a square foot today are few and far between. "Everyone I talk to says the market is so good that it's scary," he said. Puglisi added that he continues to be pleasantly surprised by the number of good restaurants near the beach doing business year-round.

The boom is not restricted to Ocean City. Similar upbeat reports are heard from the once-somnolent Bethany and fashionable Rehoboth beaches to the north in Delaware.

At Bethany, realtor Joseph R. Tansey said activity is "unbelievable . . . . It seems almost everything will sell." Despite recent price increases, the nine-building Sea Colony high-rise complex is almost sold out. Tansy said that units at Sea Colony that sold a year ago for in the $67,000 range now resell for almost $90,000. Rental of a one-bedroom unit at Sea Colony will cost $500 a week in August -- if units are available. Only half a mile island, rentals of four-bedroom Bethany West houses now cost $450 a week and the few resales here are in the $65,000 range.

Another aspect of the increasing interest shown in purchases of beach property in Maryland and Delaware is that growing numbers of owners do not rent their places to others, despite the tax advantages. It is estimated that only about 50 percent of the nearly 20,000 beach-related dwellings less than three hours from Washington are now available for tenacy.

At Rehoboth, which has more than a hundred year-round, estate-like homes in the Pines area away from the beach, rentals continue actively. There are hundreds of apartment, small house and town house rentals available every season. Realtor Virginia Joy, who has had her own brokerage in Rehoboth for 13 years, cited an increase in preleasing this year. She said the rental range is from $250 to $750 a week and that choice is slim except for weeks in June and September.

In terms of sale prices of homes, north of Rehoboth's modest boardwalk are dozens of contemporary houses costing more than $200,000. The cachet of Rehoboth has been reinforced since the venerable Henlopen was rebuilt into a new apartment-hotel building several years ago.

In summary, the popularity of Maryland-Delaware ocean-related properties for vacations and weekend visits is on the rise. "And now we get people who used to go to the Jersey beaches and Virginia Beach," said Ocean City Mayor Kelly.

Including rent, food (which costs more at the beach), incidentals and at least one night out at a restaurant, it usually costs a family of four at least $800 to spend a summer week in a beach place. But real estate broker Moore of Ocean City points out that oceanfront condo apartments in Florida would be 30 percent more expensive to purchase then in Delaware and Maryland. w