TWO MILLION TRAVELERS will go to sea in cruise ships during the 1979-80 season, an increase of 800,000 over last season. The majority will visit the Caribbean islands on one-week sailings.

This optimistic projection, based on unusually heavy advance bookings through January 1980, comes from cruise line officials and their organization, Cruise Lines International Association (CLIA) with headquarters in both New York and San Francisco.

The 2 million will be repeaters, dropouts from the expensive European vacation scene and first-timers of various backgrounds and ages. All are expected to take advantage of the increased number of fly/cruise packages which for a set price cover everything: flight from hometown to departure port, transfers to ship, baggage handling, the cruise and sometimes even port taxes, plus a return flight home.

However, don't get the idea that a cruise is a low-budget tour -- even though the price includes your cabin, food, transportation and entertainment. Per diem rates average out to from $74 to $120, per person, double occupancy. The variety of lines, ships and cruise itineraries means there is a wide spectrum of prices and services, ranging from quality to merely satisfactory. Generally speaking, a sea vacation is not cheap but it does offer value.

Making the proper choice is the key to a fun-filled cruise instead of one that may be a complete flop. It all depends on your needs and expectations. Decide on the length of a trip. Short or mini-cruises appeal to a less affluent, informal, young and active crowd that's big on the sea but not the niceties. The food, while satisfactory, can hardly be classified as "gourmet." It's fast food quickly defrosted and served cafeteria-style. And you'll rarely get full meal service in your cabin. If this doesn't bother you, fine.

However, if you prefer at least a touch of those fabulous dinners you've always heard about, look to the longer cruises and to the nationality of the ship and crew. They often produce some of their countries' best cuisine.

Longer cruises tend to mix formality with informality, giving passengers a choice. There are still many folks who like "to dress up" for the ship's nightclub entertainment.

Another clue to what a sailing will be like is the "theme" cruise. A singles cruise is not for the middle-aged and married. If you're a hobby fan and find a cruise keyed to your specialty or one you'd like to try, then book the trip.

These cruises sometimes highlight entertainment, but most ships have lots of public lounges with live entertainers. Others include a disco or two. If a ship has both, then you simply enjoy your favorite. If a ship only goes disco, you may want to reconsider and shop elsewhere.

If you don't know whether or not a ship has discos, public lounges or what, ask your travel agent for the deck plan. Learn the symbols and see where everything is located. Your cabin could be above or below the disco and the beat will go on and on . . . until you want to jump off. Not all ships have sounproofed their discos.

The deck plans will also indicate whether your cabin is outside (with porthole) or inside. Since all cruise ships sailing from U.S. ports are completely air-conditioned, an inside cabin is not much of a problem -- unless you MUST be able to look outside. It costs less, too.

Whether inside or outside, consider the size of that cabin. Newer ships have more compact accomodations. The deck plans give a good idea of size. That color photo-illustrated brochure could be misleading. Usually taken with a wide-angle lens, the picture depicts a big cabin when the contrary may be true. Note the number of chairs and closets in the photo (deck plans show this, too). The less chairs and closets, the smaller the cabin.

Another part of your cabin is the bathroom. Most have only a shower and toilet. If you want a tub, ask for it in advance.

Accomodations may also figure in whether or not you'll experience sea sickness. Most ships today have stabilizers which smooth the ship's movements except in very rough seas. Some folks, especially first timers, may become uneasy -- even if the reasons are mostly psychological. When you book your cruise early, try to obtain a cabin near the center of the ship where there is less motion rather than at either end.

After accomodations, consider service. A good guideline is to find out the size of the ship's crew. Excellent service and attention comes from a large service staff, based on the ratio of staff to passengers (usually 75 percent). Ask if the crew speaks English, another important plus factor.

Still another question to ask your travel agent is how many passengers the ship carries on a cruise. Then you'll have to decide if you want a "rush-hour" cruise or a less-peopled one. A tip-off can be whether there are two seatings for each meal or one.)

All ships schedule a full day of events when at sea, as well as first-run movies, which you can enjoy or ignore. For many, just being at sea is enough -- they want only to relax. If different ports of call are important to you, ask about them. How many does the line you're interested in visit? How long does the ship stay in each port and what time of day or night does it reach each port?

All these questions should be put to an agent who knows cruises from personal experience. Once you get the answers, book your passage. You'll see a wide range of prices from minimum to maximum, usually based on two persons to a cabin, which may not include the fuel surcharges being levied. More cruise lines are charging special singles fares (one person) that are higher than the double rate. Minimum-priced cabins usually are sold immediately. If you insist on having one, ask to be put on the "wait list."

Travel agents bonded by the International Passenger Ship Association or the Pacific Cruise Conference are your best bet because of their experience. And a federal law now protects you against loss of the cruise fare paid to any cruise line operating in this country if the line happens to go out of business before you sail.

On a cruise vacation, as the industry puts it, all costs are "up front." A passenger knows exactly how much the trip will be and can allow for extras. There is also the security offered by a cruise and the minimum of hassle, like not having to go from hotel to hotel lugging suitcases and unpacking.

Cruise rates are only slightly higher than last season, despite basic fuel surcharges of $5 per person per day. Exception is Holland America's fuel surcharges of $3 to $10 per person per day, based on cruise selected and ship.

Although most cruise lines are attempting to stabilize prices through the end of the year and mid-January 1980, prices will soar after that. For example, Cunard has just announced increases for the QE2 of 10 percent in 1980. This is in addition to 10 percent already announced this year. Royal Caribbean Cruise Lines of Miami will raise fares Jan. 12 by $50 per person, and Sitmar Cruises of Los Angeles will add 10 percent for the first quarter of 1980 and 14 percent more for the balance of that year.

Price increases are in effect already for transatlantic fares on the QE2, ranging from $110 to $150 per person, plus an extra $7 a day per person fuel surcharge through the end of October. The ship will make 23 crossings in 1980, less than her usual run, while Soviet vessels and the Polish Ocean Line's Stefan Batory will increase their departures from both Montreal and New York.

The focus, however, is more on cruises rather than crossings. Many wonder if there is still a chance of getting aboard a ship this fall. It's tight but not impossible."

Bob Dickinson, senior vice president-sales and marketing for the Miami-based Carnival Cruise Lines, suggests "that your travel agent call our line or any cruise line 30 days prior to a sailing or even as late as six days before departure. Despite the rush and less-desirable cabins, you have a chance of being aboard. Cancellations are a way of life. It's like showing up at a New York theater box office and buying a ticket for a hit show at the last minute."

Once aboard, what changes in ports of call and services await? There will be little change in itinerary until January, when some ports will be dropped and some ships will travel at a lower speed to save fuel. As to service, all the lines queried are improving services to include more entertainment, special meals, theme cruises (cooking lessons, jazz, compatibility seminars, etc), and all are striving for neat and sanitary ships to meet the strict requirements of the U.S. Public Health Service. The lines want healthy, happy passengers to return.

Watch for expanded schedules, too. Such companies as Home Lines, Holland America and Norwegian America are featuring more sailings to meet the demand. Home Lines, for example, starts its Caribbean season from Port Everglades earlier than December and is promoting it as "Autumn Economy" sailings at comparatively lower rates. Expanded schedules will mean extra Christmas-New Year's trips, always a peak season and difficult to book.

Cruise fans will also be able to try some ships new to the Caribbean. Paquet is bringing the Calypso to Miami on Dec. 16 for one-week trips. The Italian Line Cruises International introduces the Calileo Galilei from Port Everglades on Oct. 20, also on weekly sailings.

To ease the demand for berths, several new ships will enter service this fall and early winter with additional vessels to be added in 1980.

There will be more ships in San Juan, next in importance only to Miami this season as a departure port. Cunard is putting the Princess and Countess into weekly service with two different itineraries. Costa is adding the Danae and Daphne to its expansive fleet. Chandris returns with the Victoria and Britanis, which will remain year-round. Hellenic Mediterranean will operate the Aquarius.

Also new will be Royal Cruise Line's Golden Odyssey sailing between Acapulco and Miami with a stop in Key West. The Russian Kazakhstan returns to New Orleans and will add trips to the Caribbean and Cuba from Tampa, Fla.

While Miami and San Juan are now the top two departure ports, Port Everglades and New York will be third and fourth, respectively, on the East Coast this season followed by Los Angeles and San Francisco on the West Coast, with New Orleans and Galveston, Tex. (Sun Line Cruises uses this port exclusively and will put its two other ships in San Juan) sandwiched in between.

Although the Caribbean leads in cruises at this time, there's strong interest and bookings for both the Aquamarine and Holland AmericaS Prinsendam with extensive excursions to mainland China. The Aquamarine sails alternately every two weeks from Hong Kong and Kobe, Japan and the Prinsendam from Singapore.

A developing winter trend is the Red Sea cruise, starting in January with stops in both Egypt and Israel by ships of Epirotiki and "K"-Lines Hellenic Cruises. They will be available as fly/cruise programs in the United States.

World cruises are fewer in number with Cunard's QE2 and Holland America's Rotterdam both sailing from New York and Florida in January. Royal Viking Line will have one in January out of Florida.

What's ahead? Norwegian Caribbean will start weekly Caribbean service early in 1980 aboard its Norway (formerly the France). Royal Caribbean will send the Nordic Prince to Helsinki in February to be stretched. The United States, famous ship of the 50's, hopefully will clear all hurdles and sail late in 1980 from the West Coast to Hawaii and around the world.

Beyond 1980, Carnival Cruise Lines is building a fourth "Fun" ship in Denmark to be ready by late 1981 and enter service in 1982, sailing from St. Thomas. Sitmar Cruises' third ship, the Fairsky, will also see service in 1980 from Florida, and Norwegian America's Sagajford will be enlarged. The cruise industry is on a positive course.

Most lines queried were doubtful that they would soon face a full-blown recession, but Costa Cruises president Pier Giorgi Costa was seriously worried about the future of cruising. His concern was based on the "shock" of the new capacity being introduced.

Costa said his own line is concentrating on cost control and other measures so as "to be ready for the bad times to come." However, though he said he did not feel pessimistic about the long-term future of cruising, he predicted that in the 1981-85 period the North American market would be unable to absorb the 30 percent increase in berths now being planned, especially in the Caribbean.