Every Sunday, newspaper travel advertisements bring a reminder that cruise ships have armed themselves for competitive battle.

For example, seven-day cruises to the Caribbean that normally cost $1,800 are discounted to $1,000, airfare included. Those who shop around can pay as little as $700.And more is on order. This year, 13 ships with 11,000 berths are being added to the cruise line fleet operating primarily out of North America, expanding capacity by 15 percent.

Common sense would indicate that rapid expansion and the onset of a recession guarantee that the industry will sink.

But cruise line executives have not built their fortunes on pessimism and doubt. Indeed, some say a recession will only add to the business coming their way.

Bob Dickinson, a senior vice president for sales and marketing at Carnival, said that travelers will be attracted to cruise packages because they usually are all-inclusive - air fare, lodging and food.

"In tough economic times," he explained, "people want to know what they are getting into and how much they have to lay out."

Through the last decade, the outlook for the cruise business has been as sunny as the Caribbean skies.

Although most companies are private and need not release figures, analysts say industry revenues and profits have been growing consistently.

Industry executives say they have just begun tapping the market. Dickinson estimates that only about 6 percent of all American vacation travelers have tried a cruise.

The Cruise Lines International Association, a trade group, says the market for cruises in North America could reach $55 billion to $60 billion a year - 10 times the level of sales today.

Growth is limited only by space on board, said Michael G. Mueller, an analyst for Montgomery Securities.

Cruise lines "have always been able to fill up the new capacity," he said. Demographic trends are also on the industry's side, he added. "As more people get older," he explained, "they have more disposable income."

One other favorable current is the weak dollar. Some travelers, discouraged by the cost of visiting Europe, might settle for a Caribbean cruise instead.

Of course, not everyone believes the industry can sail through a recession, especially not smaller operators with aging ships.

Larry Fishkin, president of Cruise Line Inc., in Miami, which buys wholesale blocks of space then retails the berths to passengers, said that several small operators had called him in alarm over low bookings.

Fishkin can offer little comfort. Autumn is always a slow season, with price breaks the norm, but he has found that discounts are especially deep this year.

"I've seen prices for this period as low as I've seen them in seven or eight years," he said.

Carnival was offering a rate of $695 for a seven-day cruise to repeat customers, he said.

Sally Smith, an analyst with Alex. Brown & Sons, said that the industry is being squeezed from two sides.

Cruise operators, who usually buy plane tickets in bulk, are unable to pass along the entire 15 percent fare increases levied by airlines since August, when the Middle East crisis sent fuel prices soaring.

The ships' own fuel costs have risen, too, but these usually account for less than 5 percent of their expenses.

Ms. Smith said that the demand for cruises grew straight through the last recession, in 1982. But she added that the industry then had far fewer berths to fill, and that few new ships were built during the recession.

To carry the expected traffic this time around, and provide more diversions, the latest ships are also some of the largest.

The industry has been spending lavishly, on ships that can cost more than $200 million each.

Industry analysts say the berth count will keep growing, by about 13 percent in 1991 and 9 percent in 1992 before retreating to a 4 percent gain in 1993.

Possible overcapacity is unlikely to affect anyone's plans in the near term. It takes three or four years to build a ship, and halting construction is difficult and expensive.

The cruise industry has changed immeasurably since the era of the really grand ocean liners.

Back then, a cruise was the first-class way to reach a holiday destination.

As recently as 20 years ago, about 500,000 people annually, most of them rich, took cruises of three days or more.

Now, the ships are destinations in themselves. Several lines run cruises to nowhere, with passengers staying on board.

About 3.5 million people are expected to sail from North American ports this year.

The man largely responsible for lowering the gangplank to the vacationing masses is Ted Arison, who founded Carnival Cruise Lines in 1972, beginning with one old ship, the Mardi Gras, during an energy crisis. To save on fuel, the ship traveled slowly, stopping at few ports.

To keep his customers happy about spending so much time on board, he installed a casino, a disco, a movie theater and two nightclubs. Who had time to long for Caribbean ports?

Low prices and aggressive marketing helped persuade the middle class to clamber aboard.

Carnival's acquisition of the Holland America Line last year raised its revenues to $1.15 billion.

The four other companies that, with Carnival, dominate the North American industry are Kloster Cruise Ltd., which owns Norwegian Cruise Lines, Royal Cruise and Royal Viking; Princess Cruises; the Royal Caribbean Cruise Line, and Chandris Fantasy Cruises.

Next for the industry? Estelle Lessack, for one, whose Travel Trends agency in Fort Lee, N.J., handles cruises on most lines, said the idea for most is to have a cruise for every taste.

"So much is being offered on the cruise ships that it just about takes in everybody," she said.

With extra capacity visible on the horizon, the cruise ships are piling on the attractions, adding more sports, health and social programs than ever.

Norwegian Cruise Lines boasts that golf and tennis pros will help passengers with their games. Net-draped driving ranges are available on board and golf courses and tennis courts, on shore.

Others offer "spa cruises," with special weight-loss, cholesterol-lowering diets and fitness programs in elaborate gyms.

For sports fans who want to see the stars in the off-season, Norwegian also plans to bring football and baseball players on board special cruises next year.

Cruise lines are also trying to earn more from passengers once they board by multiplying the opportunities for them to drink, shop and gamble. Many spend about $50 a day, which helps compensate for cruise discounts.

To spread the news, advertising budgets are substantially higher. And then there is one more audience the lines try hard to satisfy. They make sure that travel agents have special incentives to recommend a cruise.

Besides requiring minimal effort to book, the commissions, at the larger agencies, tend to be 13 percent to 15 percent, 3 percent to 5 percentage points higher than for airlines and land-bound hotels.