Fred Astaire and Ginger Rogers lean on the rail, watching moonlight sparkle across the waves. An imperious couple swathed in deck-chair blankets summon forth another pot of tea. Elitists wearing formal clothes peer down their noses at scruffy hoi polloi in tourist class.

Such were the images of ocean travel before the jet age kicked in and the cruise business took an egalitarian plunge into the vacation industry.

Before, ships exhibited distinguishing characteristics, and the knowledgeable traveler could easily discern the differences between a Normandie and a France, a Queen Mary and a Stefan Batory.

Now some major cruise companies battle for customers by offering mega-ships that hold up to 3,000 passengers. They surround their guests with a dazzling menu of amusements and endless opportunities to spend money.

Destination: shopping

These ships, some already in service and some coming on line this year and in the near future, aren't always so easy to tell apart. They not only might be mistaken for one another; the huge cruisers could pass themselves off as resort hotels and shopping malls.

The vessels still go places, but in some cruise-line brochures, destinations themselves seem to rank in importance somewhat below slot machines and in-cabin satellite television. One glossy page after another glorifies shopping atria, the pulsing neon of the showrooms, the exquisite cuisine, the pool-side revelry. Only an occasional photograph of a palm tree or a medieval castle indicates that the mega-ship might have a destination.

The glory days of the ocean voyage, so filled with glamour and status-lust, featured mega-ships, too. One of them, the Norway, nee the SS France, still plies the eastern Caribbean for the Norwegian Cruise Line. At one time, it was the largest passenger ship in the world, and its specifications still serve as a rough definition of a mega-ship: 76,000 gross tons, a length of 1,035 feet (imagine the Chrysler Building tipped on its side) and room for 2,032 passengers.

But even with its meticulously restored art deco public rooms and first-class fittings, the Norway clearly belongs to the olden days of ocean voyages, when travelers expected lavish pampering and yet saw the line as a means of transportation. Inside the Norway, one still has the sense of riding a ship, rather than roaming through a glitzy metropolis.

In 1988, Royal Caribbean Cruises Ltd. introduced Sovereign of the Seas and snatched the "world's largest" title from the Norway with a behemoth holding 2,280 passengers. Two virtually identical Royal Caribbean ships, Monarch of the Seas and Majesty of the Seas, soon followed Sovereign on the Caribbean routes.

Although at 880 feet they fall short of the Norway's length, Royal Caribbean claims the three ships can look the Statue of Liberty (151 feet tall) right in the eye. And their bulky dimensions have been packed with amenities: five-story atria, fitness spas, theaters and showrooms, shops, children's playrooms, dining rooms, ballrooms, discos, cafes and lounges -- none of them any more nautical than those in a suburban shopping center.

The uninitiated might assume that mega-ships represent a new high in vacation insularity, an effort to cater to the unadventurous and reassure the timid with familiar surroundings. This could be considered the ideal cruising style for those vacationers who worried so much about the Persian Gulf war that they canceled their trips to Cancun.

But the story proves to be more calculated and complicated than that. An egotistical high-rise developer might risk millions constructing the world's tallest building. Cruise lines tend to be more pragmatic.

According to Rod McLeod, Royal Caribbean's executive vice president for sales and marketing, the company had little interest in launching the world's largest cruise ships simply for bragging rights. Originally, says Mr. McLeod, Sovereign of the Seas designers envisioned a 50,000-ton ship for 1,800 passengers. Then the returns from extensive market research began coming in.

"A ship has an economic life in the range of 30 years, so you want to define what your market will be -- not only today or next year, but how it will evolve," Mr. McLeod says. "This planning began around 1985, and we were looking particularly at baby boomers, young boomers as well as mid-range boomers. We began to pick up some very strong currents running through our research.

"People wanted variety in their vacations, and they wanted choice, and they wanted it on their own terms. They wanted a lot of things to do, but they wanted to pick when they would do those things. When you're looking at a land-based resort, that's a fairly simple thing to accomplish. For a ship, you begin talking about space and the amount of it you need to increase the variety of activities and choices."

But it seems those boomers wanted personal space, too, and not the kind provided by the typically claustrophobic economy-rate cabins that even the mega-ships offer. Those surveyed said they welcomed congeniality, but they also looked for intimacy -- areas on deck where they could hide.

To fulfill the growing wish list, Royal Caribbean officials asked their designers to add another fire zone to the original Sovereign plan. Ships are built in 40-foot increments, called fire zones, so a fire in any one section can be isolated from the rest. Thus, the 840-foot Sovereign of the Seas grew to 880 feet on the blueprints, primarily, says Mr. McLeod, so those boomers can get away from it all when the shipboard excitement becomes too much for them.

A place to read

"When you're on board the vessel, there is a feeling of openness and spaciousness and not a crowded feeling," he says. "If you want a quiet spot to go read a book, out in the sun or in a comfortable, air-conditioned lounge, there's always a place."

Members of the cruise industry try to make the mega-ships sound as if they can be all things to all travelers. By the time travel agent Nancy Kelly, president of Kelly Cruises in Oak Brook, Ill., lists all the demographic groups to whom the huge ocean liners might appeal, she includes darned near everyone.

"People who have cruised before love the idea of new ships that offer them even more varieties of different types of entertainment going on simultaneously," Ms. Kelly reports. "They like having great big spas on board. Children's programs can be accommodated on the larger ships very well. People like having a variety of swimming pools."

She also sees the giants as a choice for those who would rather not board a ship at all. "The great big new mega-ships ride so beautifully that sometimes you have to look out the window to see whether you're at sea or you're docked. That's a real comfort level for a lot of people."

In 1996, Carnival Cruise Lines expects to take delivery on what will be the largest passenger ship of all, an as-yet unnamed 96,000-ton vessel for up to 2,600 passengers. Sensation, a slightly smaller Carnival ship for the same number of passengers, made its debut last year, and two nearly identical sisters, Fascination and Imagination, will appear this summer and in 1995.

"It's a natural evolution of the business that we're in," says Carnival president Bob Dickinson.