Only 3 percent of Americans and Canadians -- about 8 million people altogether -- have ever taken a vacation cruise, but many of those who do have become addicted, weighing anchor every year or even more frequently. Their passion for sea escapes has not only kept the cruise industry prospering for years despite recessionary times, but it also has pushed the cruise lines into an explosive expansion.

The upshot is that instead of looking primarily to cruise aficionados, the increasingly competitive ship lines are for the first time making serious overtures to the other 97 percent of the potential U.S.-Canadian market. As a result, the cruise business for the foreseeable future will be awash in rebates ($200 from one line) and credits for future travel; guaranteed fares through 1983; and other bargains, including special discounts for families, singles and standbys.

Among the biggest savings; free or reduced round-trip air transportation to embarkation ports and free hotel accommodations for before-or-after-cruise vacation extensions.

Twenty-eight major cruise lines with 86 ships serve the North American market. Options range from overnight "cruises to nowhere" to months-long around-the-world excursions, but most cruises fall into the seven-to-14-day category. Caribbean and Mexican cruise itineraries are especially popular during the fall and winter months.

About 1.4 million American and Canadian passengers book cruises annually, but that figure is expected to grow to more than 2 million by 1985, says Ralph Bahna, Cunard Line's top executive in the United States and chairman of Cruise Lines International Association, the trade group representing the cruise companies. The lines are courting that growth with aggressive promotion and enticing price incentives.

"Consumers can expect to see really attractive cruise pricing throughout the wnole industry this fall and well into next year," says Bob Dickinson, senior vice president of marketing for Carnival Cruise Lines, "because for the first time in five or six years, supply exceeds demand." Carnival kicked off the expansion scramble in 1978 by announcing plans to build a new 1,022-passenger, $100-million vessel, the Tropicale.

The Tropicale, which entered service last January, was the first new cruise ship built since 1974 and the fifth largest passenger ship ever. It has joined Carnival's three other ships in operating seven-day cruises from Miami to a variety of Caribbean ports. (Beginning Sept. 19, the new ship will shift to Los Angeles to begin seven-day cruises to the Mexican Riviera during the fall and winter.)

Despite some fears of a capacity glut, the cruise lines will be spending nearly $2 billion by the end of this decade to expand or upgrade their fleets. In addition to the Tropicale, 10 other new cruise ships have been commissioned. One of them, the Atlantic (Home Lines), is already in service. It began Bermuda cruises from New York last April and will shift to Fort Lauderdale on October 16 for a winter in the Caribbean.

Two more, Song of America (Royal Caribbean Cruise Lines) and Scandinavia (Scandinavian World Cruises), will enter North American service by the end of this year. Song of America will debut in December with seven-day cruises from Miami; the Scandinavia sets sail Oct. 2 as a luxury liner providing car-ferry service from New York to the Bahamas and Florida.

Like the Tropicale, the new ships have $100 million-plus price tags and will carry 1,000 to 1,400 passengers, which is a departure from the 600- to 800-passenger ships that have been the cruise industry's workhorses for many years. They have been designed specifically for pleasure cruising instead of point-to-point sea transportation, unlike most existing cruise ships, which were converted from transport to cruise service as the industry developed.

Their greater size means more amenities and activities as well as more spacious and comfortable accommodations, including king-size beds, instead of upper berths and windows instead of tiny portholes. The new ships also benefit from advanced technology, such as engines that run on lower-grade, less-expensive fuel and satelite link-ups that provide passengers with direct transmission of televised sporting events and stock market prices.

Cruise lines also are refurbishing and overhauling existing ships, in cluding literally inserting new midsections in ships that markedly increase their capacity. And new cruise lines with totally new cruise concepts are emerging, such as Scandinavian's car-ferry service.

The result: With all those extra staterooms to fill, cruise lines through their advertising increasingly will be talking to consumers about prices, comparing cruises with other types of vacations. They especially will stress the all-inclusive nature of cruise costs (transportation, food, entertainment and activities). Says Carnival's Dickinson:

"Romantic pictures of ships silhouetted against palm trees are adequate to stimulate repeat cruise passengers but not the first-timers that we're all going after these days. That's why you'll see more and more price comparisions, hard facts and bargain promotions," instead of the traditional soft sell.

Here are some examples of the kinds of cruise bargins currently available:

* Free round-trip air transportation to embarkation ports: Flights to Fort Lauderdale, Miami, Los Angeles, New York and San Juan are available from a number of cruise lines, including Carnival, Cunard, Home Lines, Norwegian American, Norwegian Caribbean. Paquet Cruises, Royal Caribbean and Sitmar Cruises.

* Reduced airfare to embarkation ports: Virtually all cruise lines not offering free airfare have packages that provide significant discounts on the cost of the flight. For example, American Hawaii Cruises, which operates the Constitution and Oceanic Independence in seven-day cruises throughout the Hawaiian islands, offers its passengers a $495 round-trip air fare from Washington to Honolulu. That's significantly below the cheapest currently available excursion fare of $647.

* Rebates: Home Lines is offering a $200 rebate to passengers booking seven-day cruises from New York to Bermuda through Nov. 13 aboard the Oceanic.

* Cash incentives: Certificates redeemable for $250 to $1,500 on current and repeat cruises are available from a number of cruise lines, including Norwegian American, Princess Cruises, Royal Viking Line and Costa Cruises.

* Flat rates: The cost of a cruise is normally determined by the type of cabin selected, with accommodations available in various price ranges depending on their location and level of comfort. (All passengers, no matter what type of accommodations they have bought, enjoy the same dining and recreational facilities on board a cruise ship.) Now, however, Commodore Cruise Line is offering flat rates for all cabins except suites on the Boheme's Caribbean sailings.

* Standby fares: Holland America Cruises is offering Impulse Cruises at a rate of $795 per person for a week-long sailings from New York to Bermuda aboard the Veendam and Volendam. Passengers pick a first, second and third choice of departure dates but may not know when they are leaving until two weeks before sailing time.

* Family/Singles discounts: Third and fourth passengers sharing cabins travel at significant discounts, and sometimes free, on many cruise lines. Some lines offer guaranteed rates for single passengers willing to share a cabin with a stranger in order to obtain the double-occupancy rate. If the line cannot find a suitable roommate, the passenger will get a single-occupancy cabin but at the lower rate.

* Free hotel accommodations: Vacation-stretching options of up to seven nights free hotel lodging (plus free sightseeing or rental car in some cases) are available in such destinations as Florida, San Juan, Barbados, St. Lucia, Honolulu and London. Costa, Cunard, Princess and American Hawaii Cruises are among the lines offering these add-ons.

*Introductory prices: Scandinavian World Cruises is offering a free return trip to passengers who book its unique New York-Bahamas-Florida cruise/ferry service, which begins in October. The new cruise line is targeting the 3 million people who annually drive from the Northeast to Florida, offering to carry passengers' cars along with them for free. The line's flagship, the Scandinavia, which has a capacity of 1,600 passengers and 400 cars, will take 2 1/2 days to sail from New York to Freeport, Grand Bahama, at a one-way cost of $330-$550 per person. In Freeport, Florida-bound passengers can transfer for a charge of $59 to one of the line's other two ships, the Scandinavian Sun and the Scandinavian Sea, which make daily round trips to Miami and Port Canaveral, respectively. Vactioners can sail back to New York free if they don't want to drive home.

To get details on these and other cruise bargains, travelers should seek out a knowledgeable travel agent who can handle all inquiries and bookings at no additional cost. The individual cruise lines can also be contacted directly for information about their ships.

And, for novices considering a cruise vacation, Cruise Lines International Association has a fact-filled booklet that describes and explains what cruising is all about. To get a copy of "Answers to the Most-Asked Questions About Cruises," send a stamped, self-addressed, business-size envelope to CLIA, Pier 35, Suite 200, San Francisco, Calif. 94133. CAPTION: Picture, From the Cruise Ship Project, Copyright (c) Barry M. Winiker