The cruise industry, an uncertain if basically healthy 20-year-old, is increasing its efforts to change its image. Once thought to be the playground of only the idle rich, cruises are now being aimed at families, young couples and singles, and at personalities as varied as the scholarly, the athletic and the adventurous.

The cruise industry wants nearly everyone, in fact -- including the rich.

In an effort to broaden their appeal, cruise lines in 1986 are adding new types of cruises, new destinations and, most significantly, new ships. In the meantime, the industry is also coming to grips with two very different kinds of problems: the threat of international terrorism and an inability to fill its ships completely.

Terrorism, which struck directly at the cruise business with the October hijacking of the Italian ship Achille Lauro, has caused the lines -- particularly those that sail in the Mediterranean -- to reevaluate their security measures and in some cases drastically revamp their schedules.

The abundance of empty berths results from an ongoing rapid increase in new vessels, giving the industry a current capacity rate of only 85 percent. This overcapacity will burden the lines with slimmer profit margins and an undependable future, unless the market picks up as much as expected. Until then, those unfilled berths can mean a financial edge in favor of the passenger, as the lines try to get more customers by discounting their rates.

There's another reason for taking a cruise in 1986: at $160 to $220 per person per day, average real prices are among their lowest in five years, according to industry figures.

"We are going from one stage of growth to another," says Ron Zeller, chairman of Cruise Lines International Association (CLIA) and the president of Norwegian Caribbean Lines.

"When the airline industry first brought on wide-bodied jets in the form of 747s, there was a lot of speculation that their greater capacity would be underutilized. Instead, the opposite happened, and the airlines boomed. The cruise industry is ready for the same type of growth."

But at the moment, discounting to fill empty berths is a necessity for cruise lines. "The discounting is a means of saving for the consumer, there's no doubt about it," says Charles Bressler of Cruises Unlimited, a Chevy Chase travel agency. "However, the steamship lines are reluctant to advertise until the last minute that they have a lot of cabins available. They don't want to educate the consumer to wait."

In looking for a better deal, in other words, you could end up missing the boat entirely. The biggest discounting advantage is achieved by groups, where the travel agent, because of the number of berths at stake, has some leverage with the line. A group discount, Bressler says, can range from 5 to 40 percent, depending on season and ship.

"Chances are very slim the line will give an individual a discount," says Bressler. The best hope for a better deal for individuals is if their requested passenger class is unavailable. The line may then guarantee that class; if unable to provide it through a cancellation, the ticket will be upgraded instead, at no extra cost.

Other methods of getting a price advantage include the advertised "early bird" specials -- with discounts for booking way in advance; traveling with friends or family and getting a cabin that can accommodate a third or fourth person at reduced rates; and contacting a short-notice clearinghouse such as Worldwide Discount Travel Club (1674 Meridian Ave., Miami Beach, Fla. 33139, 305-534-2082).

"Buyers of such tickets should check out their clearinghouse before they hand over any money," warns Ted Hankoff, president of Worldwide. "We tell our members to call our bank if they have any questions, and we say we'll provide them with references, and that we're bonded and licensed."

Worldwide, which charges a fee of $45 a year per family, sends a list of available trips to members every third Friday. Cruises of seven days or less can be booked as much as 30 days in advance; on longer cruises, it's as much as 60 days. Discounts, Hankoff says, are from 15 to 65 percent.

Beyond the financial strain of discounting, another factor weighing on the cruise industry -- as, indeed, it is on the whole travel business -- is terrorism. Since the Achille Lauro hijacking and in the wake of other terrorist incidents in Europe, several lines are changing their eastern Mediterranean schedules. Says one industry spokesman: "Everyone's either moving west or moving out."

Among those moving out, at least temporarily, is Princess Cruises. The line had scheduled its Pacific Princess (TV's "Love Boat") for the Mediterranean from this April to November. Instead, the ship will be based in Seattle.

"Frankly, our booking level for the Mediterranean this coming season was so low compared to a year ago that it was abundantly clear that, irrespective of any advertising, the public was not interested in going to that area in 1986," says Michael Hannan, senior vice president for marketing at Princess. "We're hoping that that situation should turn around in 1987."

Other lines with changed schedules include: Royal Viking Line, which has reduced its cruises in the Mediterranean from 10 to three this year and is placing those remaining three in the western Mediterranean; Royal Cruise Line, which has canceled 19 Mediterranean cruises, keeping only three; Sea Goddess, which has dropped cruises to Egypt and Israel, cut visits to Rome and Athens by 40 percent and is now concentrating on the western Mediterranean; and Windstar, which had planned to put a new ship in the Mediterranean in 1987 and is now reconsidering that idea.

"This is not a situation that's been taken lightly by the cruise industry," says CLIA chairman Zeller. "I think every line has evaluated its security measures in the past year and has made changes, whether they operate in the Mediterranean or elsewhere."

Among tightened security measures mentioned by the lines: curtailed visitation or no visitation at all; use of metal detectors and X-ray equipment similar to that employed by airlines; bomb-sniffing dogs; requiring identification to gain access to the ship; additional officers on duty at the gangway; and increased security in and around docked ships, including at the home port.

The cruise lines caution, however, that the threat of terrorism can be overemphasized.

"The best risk assessment I heard was by a RAND think-tank security expert," says Zeller. "When asked what he advised people going overseas to do, he said, 'I'd advise them to buckle their seat belts on their automobile, because they have a greater likelihood of being injured on the way to the airport than they do on the trip itself.' "

The idea of cruising for fun -- instead of simply as a means for getting to and from Europe -- is barely two decades old. More than 2 million people are expected to take a cruise this year, up from 1.8 million in 1984 and 500,000 in 1970.

The potential audience, however, is considered much larger: an estimated 35 million to 40 million people. To capture these travelers, the industry is experimenting with generic advertising on television, presenting cruises as relaxed, informal and fun, eminently suitable for young couples and families with kids.

To eliminate concerns about cruises costing too much and requiring too much time, shorter (and therefore cheaper) cruises are being touted, while to combat what some saw as a too-relaxed atmosphere, there's more name entertainment, more theme cruises and a greater emphasis on activities and sports.

"Most people have an inaccurate image of cruise ships: The Titanic and 'A Night to Remember,' trans-Atlantic crossings, steerage, too much food, rich people cavorting," says Diana Orban, who does public relations for CLIA. "There's an education factor that's just enormous -- but that's changing."

Also changing is the age of the average cruise-goer. "When I started in this business, the average age of the passengers was deceased," says Bob Dickinson, senior vice president for sales and marketing at Carnival Cruise Lines.

No long-term age statistics are available, but CLIA says that even in the past few years, the average passenger age has dropped. In 1983, it reports, 25 percent of passengers were under age 34; one-third were between 35 and 54; and 43 percent were over 55. In 1985, while the middle age bracket remained constant, the younger went up to a third of all passengers and the older went down to a third.

CLIA, whose 26 member lines carried 90 percent of U.S. cruise passengers last year, claims that once a person takes a cruise, there is an extremely high chance he or she will take another, with individual lines reporting a repeat business from 25 to 65 percent.

"Our objective is to get them to take that first cruise," says CLIA president James Godsman. "So we're training travel agents, selling basic cruises, issuing promotional literature on behalf of the whole industry and doing a major television advertising and promotional test in Missouri."

Some critics argue, however, that such innovations as generic advertising only scratch the surface of what has to be done.

"The cruise industry is as if the car industry had nothing more to offer than four-door sedans," says William Kyle, a consultant working with Windstar Sail Cruises in Miami. "A lot of people want four-door sedans, but a lot of people want convertibles and pickups and everything else. That's what the cruise industry needs to be doing. Where are its sports cars?"

CLIA chairman Zeller is more optimistic.

"There are two major trends in the cruise business," he says. "There are larger ships with greater capability of providing an overall vacation package to the consumer, while the other trend is to providing specialty products and filling niches in the market."

Small luxury vessels, carrying about 100 passengers and offering deluxe accommodations, are carving out a segment of the market. At the other end of the spectrum, the largest cruise ships ever, carrying more than 2,500 passengers, are being built.

"Over the next decade, we'll have competitive marketing as well as competitive product innovation," Zeller says. "The cruise industry is embarking on a new era."