A FEW FEET under the waterline of most cruise ships is a vast "underground" area most passengers never see--a world of massive refrigerators and freezers, fork lift trucks and oversized elevators.

It is the provisions area of the ship--a remarkably well-organized, massive storage area for enough food and supplies to handle journeys ranging from one week to three months. Everything from two full glass, china and silverware services to more catsup than humanly imaginable is usually stocked down below.

When the newly stretched Royal Viking Sky left the United States for Europe a few months ago to begin its new cruise schedule, the ship carried an incredible 183 tons of food and supplies crammed into its 23 large provisions lockers.

"Most people have no idea what goes into a cruise in the way of provisions," says Gottlieb Oberweger, the Sky's chef de cuisine. Before each cruise, Oberweger meets with his staff to plan--and then order--enough provisions to meet the anticipated needs of all his passengers. Aided by an onboard IBM computer system, the chef meticulously lists 23 varieties of fish, 12 kinds of shellfish, steaks, eggs and dairy products, as well as a host of dry goods.

"It's not always easy to plan," he says, because on each cruise many passengers seem to want different things in varying quantities. As a result, the ship ultimately stocks much more than it will need for any voyage--just in case.

On a typical two-week Mediterranean journey, the passengers on the Royal Viking Sky will consume 400 pounds of potatoes and 200 pounds of meat each day. The potatoes are flown in from Idaho; the steaks come directly from Kansas. "There are certain things in which America excels," says Oberweger "and these are areas where keeping to an American standard is a must."

The passengers will also eat 2,000 eggs per day, and caviar (which the cruise line makes available to passengers whether it's on the daily menu or not) remains a particular favorite among Royal Viking veterans. Royal Viking imports its caviar from the Soviet Union in four-pound tins that cost a minimum of $430 each. On a two-week cruise, more than 14 tins will be eaten.

At each port, Oberweger goes ashore to the local markets to buy specialty items not found anywhere else. His flounder is bought in Holland; he buys Jarlsberg and goat cheese from Norway, along with a healthy supply of fresh (and delicious) reindeer meat. When the ship visits the Orient, Oberweger makes a point to purchase as many pounds of Thailand tiger prawns as the local Bangkok market has available.

On a recent Mediterranean cruise he bought sardines and mackerel in Livorno, Italy, as well as 350 pounds of fresh water mullet. In Sicily, he bought 150 pounds of mussels.

Three days later, when the ship arrived in Villefranche (on the French coast), Oberweger had telexed ahead for more mussels, escargot, oysters and a healthy supply of fresh rainbow trout.

Oberweger is usually one of the more welcome visitors to Yugoslavia. When the Royal Viking Sky docked in Dubrovnik, the merchants at the local market greeted him as the conquering hero--and with good reason. Less than an hour later, the chef had purchased literally everything they had to sell--Oberweger bought 300 kilos of potatoes, 120 pounds of sardines, 89 pounds of mullet and all the available cherries in Dubrovnik. When he returned to the ship, the local market simply closed for the rest of the day.

For a typical 28-day Caribbean cruise, Princess Cruises carries 16,000 pounds of beef; 20,000 pounds of pork, lamb and veal; 10,500 pounds of poultry; 49,000 pounds of vegetables; 44,500 pounds of fruit and a staggering 70,000 eggs.

The line boards most of its frozen food in Los Angeles; fresh fruit and vegetables are loaded in both Los Angeles and St. Thomas in the U.S. Virgin Islands. In addition, Princess goes to great lengths to fly in their lobster tails from Australia, their caviar is shipped from Russia; pate comes from France, chocolates from Switzerland.

Liquor consumption on most cruises, is, as expected, high. On a regular transatlantic voyage aboard Cunard's Queen Elizabeth 2, the ship carries 1,000 bottles of champagnes, 1,200 bottles of assorted wines, as well as 41 brands of whisky. In addition to their vast liquor supplies, the British ship carries everything from the expected 50,000 tea bags, to 50-pound bags of something called "Super Crunchies for Hungry Doggies." (Yes, Cunard is virtually the only ship allowing for the transportation of your favorite canine).

Some cruise lines, like Carnival (based in Florida) and American Hawaii (based in San Francisco) stick to the basics--7-day cruises. And their provisioning needs are more fundamental. "We've steered away from special provisioning," says Cheryl Gregorio, spokeswoman for American Hawaii. "We're an American ship and we try to respond to American eating habits."

On a typical 7-day cruise around the Hawaiian islands, the 850 passengers and 300 crew aboard either the line's Constitution or Independence will bid a bon voyage to 8,000 pounds of U.S. beef as well as 450 gallons of U.S. ice cream. The cruise line does make one provisioning exception--at some of the islands, some of the more local ice creams are stocked--everything from guava to macadamia nut.

On board Carnival Cruise Lines' ships out of Florida, fleet-wide figures reflect a weekly beverage consumption of 62,400 cans of American soft drinks, surpassed only by 67,200 bottles and cans of domestic and imported beers. In a seven-day period, Carnival's passengers will obliterate 8,640 bagels, 640 pounds of smoked salmon and 1,200 whole ducks. And, their appetites are justbeginning--they'll also cruise their way through 86,400 shrimp, 15,360 hot dogs and 16,800 hamburgers. The menu isn't totally American. The line's deck officers are Italian and they, too, have to be kept happy in the dining room. Every week, more than 5,200 pounds of pasta and 600 pounds of manicotti say arrividerci.

"We can give you these figures," says Tim Gallagher, Carnival spokesman, "because we've got it pretty much down to a science as to what our passengers will consume in a given week."

Sometimes, even the computer goofs. On a recent cruise, the Royal Viking Sky was running dangerously close to exhausting its precious cargo of beer. Oberweger telexed an urgent request to the line's agent at the next port.

When the ship docked two days later in Livorno, there were 24,000 bottles of Heineken waiting, stacked neatly on the pier. The 1,000 cases of the brew had been quickly trucked in from Amsterdam the night before.

"We were very happy to see it," laughs Lorenzo Pizzia, the ship's chief steward. "But that beer only represented a month's supply!"

Cor Stapel, purchasing director for Holland America, can't afford to run out of many provisions. For the line's 18,827-mile around-the-world cruises, Stapel starts provision planning this month for January 1984 sailings. "We spend long hours," he reports, "determining the entire package of food required for the voyage. One of our main thoughts in this planning is that we deal with almost an exclusive American clientele. They know what they want."

Often, the requirements are exasperatingly precise. "We are well aware," he says, "that you can buy tomato juice in Singapore, Italy and dozens of other places in the world. But it doesn't taste like American tomato juice. Therefore, we stock it in the U.S. only."

Stapel also will not substitute American beef (from Kansas and carried for the entire voyage), flour, lettuce and celery. (Both the lettuce and celery are specially wrapped and packed in California to last entire trip).

Certain items, like fish, milk and other dairy products, are replenished at selected ports enroute. Stapel selects the ports in advance, and the ship telexes the request to its purchasing agent a day before docking. "There are certain places," he says, "where we can't trust the quality or the freshness. We will not buy anything there under any circumstances."

However, no amount of planning can anticipate specific passenger demands, and Stapel and his staff have been known to perform some beyond-the-call-of-duty feats to meet them. "We've always had the attitude," he says, "that if the request is slightly reasonable, we'll fill it.'

Sometimes, that is virtually easier said than done. On one Holland America trip (New York to San Francisco via the Panama Canal), a woman passenger demanded a special kind of English mustard. It wasn't available on the ship, and the woman had more than just a passing taste for this particular brand. Stapel spent half a day trying to track down some of the stuff in New York, but no store carried it.

When the ship reached Panama, Stapel received an urgent telex from the captain informing him that the need for this particular mustard had now turned into a crisis--the woman was threatening to leave the ship. Stapel made more phone calls and finally located a few jars in a Manhattan warehouse. The mustard was flown directly to the ship.

Occasionally, passengers will book one cruise line over another solely because of the provisioning. "We had one passenger who had booked another ship until she found out they didn't stock Dom Perignon 1974, her favorite year," Stapel reports. "She then came to us. We checked, and found that one of our ships in Florida had 15 cases of it on board. She was elated."

However, there was a slight hitch. Although the first port after New York for the world cruise was Florida, the woman insisted she had to have the champagne for her bon voyage party in New York. Stapel happened to be in Florida when he heard the request, grabbed six bottles of the stuff from the other ship, wrapped them in a towel, threw them in a flight bag and flew up to New York where they were presented, properly chilled, in person. "Another problem solved," he chuckles.

On another Rotterdam world cruise, a passenger was particularly partial to something called fudge royal ice cream. Stapel ordered a 15-gallon tub of it put on the ship. However, he underestimated the passenger's occasional ice cream binges. Another urgent telex from the ship, this time from Singapore with the bad news that their supply of fudge royal had been exhausted. Stapel had another 15-gallon tub flown over in time to join the ship before it left for the next port.