"The Flintstones" cartoons was blasting over the television set yesterday morning, as Douglas Cosker and his sister Diane, 11 settled into their fourth game of "fish" on the living room floor. Things were getting tense.

"Hey, you looked at my cards!" screeched 6-year-old Douglas, throwing down his playing cards.

"Do you wanna play or don't ya? his sister Diane yelled back.

Their parents sat at the kitchen table, discussing - over coffee and two ash trays full of cigarette butts - what to do next with the children.

"We're getting cabin feaver, so I think we're going to take the kids to the pool today," Norman Cosker said, "even if it rains."

It was the third day of cool, cloudy, dismal weather for the families like the Coskers, who came to spend the Fourth of July holiday on the Delaware coast. They came to this quiet, family resort expecting to soak up sun and sand. But instead they found the weekend a test of their wits: for our days, they had to figure and refigure how to keep the kids - and themselves - occupied.

By yesterday, Cosker was running very low on ideas. For Bethany, a blend of turn-of-the-century seashore houses and modern town houses and villas is with plenty of places to play outdoor tennis, volleyball, shuffle-board or basketball. Cosker already had taken the kids to the movies in Ocean City on Monday, to see the "Bad News Bears."

"You'd think everybody between here and Connecticut was at the movies," said Jerrie Toombs, whose family is sharing a town house in Bethany with the Coskers for the holidays "There was an enormous traffic jam in Ocean City. It seemed like everybody was just so bored, they decided to get into their cars and drive."

And so it was for many of the Fourth of July vacationers on the beaches of the Eastern Shore, who spent much of their time this weekend in bars, in stores, in restaurants, at the movies or in front of television sets.

When the weather first began to change Sunday from hot and sunny to cool and rainy, Richard Johnson, who runs Bethany's general store - he is a veteran of many rainy beach weekends - brought out the store's extra supply of poker and pinochle cards and displayed them prominently on a top shelf in the front of the store.

Johnson, a former Montgomery County resident, who retired as a first sergeant in the U.S. Marines in 1971, says that his biggest selling items during a rainy weekend always are playing cards, games, and Junk food.

By lunchtime yesterday, the store's racks of peanuts, potato chips, pretzels, and caramel covered popcorn, were near empty.

"The Amercian people are terrible eaters," Johnson said philosophically.

Since liquor is not sold in Bethany, many of the bars, discos, and restaurants in Ocean City were able to attract a large portion of the over-18 crowd that had been staying in Bethany over the whole weekend.

Monday, when it rained heavily for much of the day, Fager's Island, one of Ocean City's best known night sports, was "busy from the time we opened (11 a.m.)." according to head waitress Patty Toye.

"It was the same faces at the bar all day," Toye said.

The Sun and Surf Movie theatre in Ocean City offerred continuous "bad beach day matinees" on Monday and had a record 18 sell-out performances that day, beating even the record for single-day sellouts that Star Wars set at the theater last summer, according to manager Joy Hayes.

It was also a "banner day" at Ocean City's roller skating rink Monday afternoon. The rink rented 613 of its 680 pairs of roller skates at the same time, according to the manager, Lee Baker.

Not all the vacationers found it hard to find things to do. Bruce Patner of Potomac, who was spending the week in Bethany with his wife, Myra, and two children, said the family "went exploring" around Bethany and discovered a canal and a pier near the house they were renting, and a place where you can get fresh fish right off the commercial fishing boats in North Bethany.

"We were disappointed about the weather, but we were determined to make the best of it," Patner added as he sat dressed in white tennis shorts and T-shirts on the stone porch of his cottage. "We got to see another side of Bethany."

Police from Bethany, Rehoboth, and Ocean City agreed that it had been a "slow, quiet," weekend for a holiday and said that a large number of visitors had left the resorts by Monday night.

"It just doesn't seem like the Fourth of July," said Devereaux Raskauskas, who's been coming to Bethany for the past 10 years, noting how empty the town seemed yesterday.