(MONEY Magazine) – Grab your Docksiders. There has never been a better time to get a deal on a cruise. With 90,000-plus berths to fill, cruise lines everywhere are offering steep discounts of 20% to 30%. In what follows, we've focused on the eight best bargains through December, but you can find deals throughout 1992. Be prepared to do some homework. ''The list price of a cruise is as fictitious as the sticker price on a car,'' says Ed Perkins of Consumer Reports Travel Letter. Cruise vacations are more popular than ever, with passengers growing from 1.4 million in 1980 to 3.64 million in 1990, according to the Cruise Lines International Association. But the industry has been adding too many berths too fast. ''There's enormous overcapacity out there,'' reports Mike Grossman of Cruises of Distinction, a cruise-only travel agency. By the end of 1992, 12 new ships will add 9,700 berths that will be discounted at their launch. The fastest-growing market right now is for three- and four-day cruises departing from ports such as Fort Lauderdale, Miami and Los Angeles. Booking one -- at $500 to $800 per person -- can give you a taste of cruising without having to commit much money or time. Family cruising is also growing -- a week for a family of four averages $3,500 or so. ''Kids can be independent on board,'' explains Jo Kling of Landry & Kling agency, ''which is a great relief for parents. Also, menus have no prices.'' How to find the cruise that suits you. Rating cruise lines is more difficult as old companies merge and new ones emerge. Nevertheless, agents agree that the deluxe lines -- about $2,000 and (way) up per person for a week -- include Royal Viking, Renaissance, Crystal Cruises, Diamond and Cunard. Those dubbed premium offer quality cruising at a more competitive price -- $1,700 to $2,400 -- and include such lines as Celebrity, Holland America, Princess, Royal Caribbean, Royal Cruise and Sun. Popular lines, such as Carnival and Norwegian, target younger travelers -- typically for $1,200 or more. And true budget lines, like Commodore, Dolphin and Chandris, pare the frills to start at about $995. Cruise-only travel agents (see the box at left) can stretch your dollars most -- 25% to 35% on out-of-season sailings and discount fares. When talking to an agent, be clear about what you want. Some people demand five ports in seven days. For others, the ship is the true destination. You can check on a ship's cleanliness by writing to the U.S. Public Health Service (1015 North American Way, Room 107, Miami, Fla. 33132) for a free copy of the ''green sheet'' -- a report that rates sanitation on all cruise ships that call at U.S. ports. How much should you spend? Luxury on cruises can be measured by the passenger-to-crew ratio on ship: 2.5 to 1 is considered comfortable, as on Royal Caribbean's Nordic Empress; 1.3 to 1 is truly pampered, found on Cunard's Sea Goddess I and Sea Goddess II. Don't jump at the cheapest fare without knowing what you're buying. Very low advertised cruise prices may not include air fare. What does ''all inclusive'' mean? Usually, the price covers your cabin, meals and air fare. Everything else is extra (see the table above), from alcoholic beverages (poolside cocktails run about $2.25 to $5.50) to ship-to- shore phone calls ($10 a minute). As for tipping, Norwegian Cruise Line is typical. It recommends tipping cabin stewards and waiters $3 a day per person and tipping busboys $1.50 a day per person. On a seven-day cruise, therefore, you can expect to shell out $52.50 per person in tips at the end. What happens if you cancel? Depending on how close to embarkation you cancel, you can lose from 10% to 100% of your money. If in doubt, cover yourself with trip-cancellation insurance, which typically runs $5.50 per $100 of coverage. A $1,200 cruise, for example, costs $66 to insure. The three * major insurers are Mutual of Omaha (800-228-9792), Access America (800-284-8300) and BerkelyCare (800-323-3153). Another option is the ''cancellation waiver'' offered by many cruise lines for a flat fee of $50 to $60. This exempts passengers from penalties normally levied when they cancel. (Some lines require a doctor's note for illness.) The season's best cruises for less than $1,200. In the cruise biz, a last- minute booking is one made three months or less before sailing. Discounts increase the closer you get, reaching as much as 60% off. The prices include round-trip air fare from major cities plus meal and cabin costs. The prices are discount fares quoted by cruise-only agencies and are per person based on double occupancy. -- Transatlantic. From September to December, buy into Cunard's 1,864- passenger QE2 luxury from New York City to Southampton, England at a bargain standby fare. You get five nights in Transatlantic Class in an inside cabin, and return from London via British Airways. Cunard Lines, $1,149. -- Caribbean. The new, 1,200-passenger Royal Princess sets sail from Fort Lauderdale through Nov. 30 for seven-day cruises, calling at Nassau, Montego Bay and Cozumel. Outside cabin. Princess Cruises, $1,000. -- Young and restless. From October through November the 2,044-passenger Norway sails from Miami for a week, stopping at St. John, St. Thomas and Pleasure Island. Outside cabin. Norwegian Cruise Line, $1,040. Mexican Riviera. Seven-day sails from L.A. aboard the 870-passenger Fair Princess, calling at Cabo San Lucas and Mazatlan -- through Nov. 30. Outside cabin. Princess Cruises, $1,040. -- Romantic. The 654-passenger Seawind Crown sails from Aruba through Dec. 8 for seven days, visiting Caracas, Trinidad and Grenada. Seawind Cruises, $1,100. -- Four days from Miami. From October through December, the 1,600-passenger Nordic Empress sails from Miami to the Bahamas. Outside cabin. Royal Caribbean Cruise Line, $670. -- Four days from Los Angeles. The 1,498-passenger Viking Serenade leaves L.A. through Nov. 25, stopping at San Diego and Ensenada. Outside cabin. Admiral Cruises, $625. -- For the family. Three-day cruises from Port Canaveral, Fla. on a Premier Cruise Line ''Big Red Boat,'' calling at Nassau, followed by four days at Disney World, October through December -- an outside cabin that can sleep five costs $910 per adult, $645 for each kid.

BOX: THE TOP CRUISE-ONLY TRAVEL AGENTS

In this climate, any competent travel agent can get you a competitive price on a cruise. But cruise-only agents are often seagoing veterans with direct knowledge of ships and discounts. What follows are the largest cruise-only travel agencies: -- The Cruise Line. Based in Miami since 1983, this is a clearinghouse for discount cruises; 800-327-3021. -- Cruises of Distinction. Besides discounted cruises, the Montclair, N.J. agency offers a catalogue for browsers; 800-634-3445.

-- Landry & Kling. Founded in 1982 in Coral Gables, Fla., the agency focuses on special-interest cruises; 800-448-9002. -- World Wide Cruises. In business for five years, this is a volume discounter based in Fort Lauderdale; 800-882-9000. -- Spur of the Moment Cruises. The specialty of this agency is last-minute bargains; 800-343-1991. For a list of others in your area, send a postage-paid envelope to National Association of Cruise Only Agencies, P.O. Box 7209, Freeport, N.Y. 11520.