It was the last night of our cruise through the Mediterranean and the Adriatic on the Royal Viking Sky. The notices had been delivered to the cabins informing passengers that bags had to be outside each room by 5 in the morning to be offloaded in Piraeus, Greece.

But hardly anyone went to sleep early. Instead, a large group gathered in the ship's casino for one last fling at the slot machines, roulette or blackjack.

I was one of them. After nearly two weeks on the ship, I was ready to win. I promptly lost $200, but my seatmate at the 21 table (an elderly woman from Philadelphia who was a cruise-ship veteran) was blessed. She won almost every hand. Shortly after midnight, she gathered up a large pile of chips and walked the short distance to the casino cashier.

"I do this every cruise," she said, smiling. "I may not pay for the cruise itself, but I never have to worry about tipping the staff."

Less than a decade ago, a casino on a cruise ship was the exception to the rule. Once frowned on by lines like Royal Viking, casinos now can be found on just about every major ship in the world that carries 300 passengers or more. Passenger surveys and other research apparently have shown that those travelers want to gamble on their vacations -- and in recent years the lines have been accommodating this need at a time of increased competition and oversupply of berths.

As a result, many cruise lines had to add casinos on ships originally built without them. Royal Cruise Line's Golden Odyssey was constructed in 1974, and the casino was added in 1980. When the company purchased the Royal Odyssey in 1982, the casino was built during the ship's refurbishment. Holland America's flagship, Rotterdam, was built in 1959, and the casino was added in 1974.

"We fought the idea for years, but the passengers kept asking for them," says George Cruys, spokesman for Royal Viking. "When we finally installed the casinos, we emphasized that we did it as an entertainment feature of the cruise, and not as a profit center."

Lines like Royal Viking don't operate the casinos themselves, but farm them out as concessions in much the same way as they offer a shipboard photographer. The cruise line provides a space for the casino and cabins for casino staff. But an outside firm actually hires the employes and runs the gambling operation.

For example, Atlantic Associates of Springfield, Mass., runs the casinos for Sitmar and Royal. London Clubs Limited of London runs the Cunard casinos. Casinos Austria of Austria runs the casinos for Royal Viking.

An exception is Princess Cruises, which runs its own casinos. They recruit their staff from northern Nevada and requirements are stiff -- a dealer must have five years of experience dealing two games -- 21, craps or roulette. A slot mechanic must have five years mechanical and some managerial experience.

The games and betting limits vary from ship to ship, and so do some of the other casino rules.

Understandably, no one under l8 is allowed in the casinos on most cruise ships. On Holland America cruises you must be 21.

And, in keeping with the fact that it is, after all, a cruise, there is no dress code in most cruise ship casinos. People wear anything from shorts during the day to tuxedos and gowns on formal nights. However, most do require that you wear a cover-up over your bathing suit.

By international law, the casinos are not open when ships are in port -- not because gambling is illegal but because the casino, like the gift shop on board, is not allowed to compete in any way with the businesses ashore. Once at sea, each line operates its casinos differently.

Sitmar's casino rooms are open all day and night, but only for slot-machine playing. If you're a blackjack or roulette player, then you'll have to wait until dinner time.

Holland America's casinos are closed while meals are being served. Otherwise they are open from 10 a.m. until 2 a.m.

On the Pearl of Scandinavia, the casino hours are listed, but only as a guide. If people want to continue playing late into the night on the South China Sea, the dealers will often oblige.

Princess Cruises' casinos are closed when the last players go back to their cabins. Pacquet Cruises, with the same policy, simply lists the closing time of their gaming tables as "late."

Most people don't choose a cruise by the type of casino aboard their ship, but if you're a diehard craps player you may be out of luck. Few ships have a table.

Sitmar has three blackjack tables, one roulette table and 75 slot machines on each of its ships. All Cunard ships and all three of Holland America's ships have blackjack and slot machines, but only one of Cunard's and two of Holland America's ships combine blackjack, slot machines and roulette.

Royal Cruise Line has casinos on both of its ships. The Royal Odyssey has four blackjack tables and about 45 slot machines. The Golden Odyssey has two blackjack tables and about 31 slot machines.

Princess Cruises' ships have slot machines out on deck (you can gamble and watch the scenery!).

One unfortunate problem does persist on nearly all the vessels -- many passengers want to gamble but don't know how to play the games.

On almost every cruise I've taken, the casino is full of people who will play 21 without sufficient knowledge of the dynamics of blackjack. Each cruise ship casino lists the rules of each game on the wall, and in a pamphlet that I've never seen anyone read.

"The object of the game is to hold a hand higher than the dealer's without going over 21," reads the sign on board Sitmar's new, spectacular Fairsky.

But, that is not the entire goal of the game. As a player, your object should be that, no matter what cards you hold in your hand, the dealer should draw cards that go over 21 and bust him. If you happen to have a good hand, that's also terrific but not essential.

Last year, on one Alaska Fairsky cruise, I watched two men lose a few hundred dollars by hitting a pair of nines when the dealer's face card was a five. They mistakenly thought the object of the game was to get 21 and beat the dealer. No one had bothered to tell them how to play, and they were too embarrassed to ask.

Occasionally, Royal Viking casinos offer instruction hours when passengers can learn to play. The casinos sometimes are also open for free play hours, when you can play without money to gain experience.

If you are going to play, you should know the casinos' betting limits and credit rules.

Blackjack on the Cunard Princess and Countess has a $3 minimum and a $200 maximum. All other Cunard ships have a $5 minimum and a $200 maximum for blackjack. On Royal's cruises the slot machines are 25 cent and $1 machines. The blackjack tables have a $5 minimum and only occasionally is the minimum lowered to $2.

There are, of course, some high-roller ships, and some cruises that lend themselves to more serious gambling. Royal Viking's betting limits soar to a $500 maximum.

Western Cruise Lines' three- and four-day Mexico itineraries from Los Angeles have attracted some heavy gamblers. A few times during the year, some cruise lines offer "cruises to nowhere" -- two- and three-day jaunts that fill the casinos.

On one recent "nowhere" excursion in Asia, a group of serious gamblers took the casino on the Pearl of Scandinavia for $10,000. "They were excellent card players and they were quite lucky," a dealer told me. "It was one of the few times we kept hoping for a storm," she said, laughing, "but the seas stayed calm."

Each cruise line can point to some big casino winners, passengers who hit the jackpot just after leaving Tunis or win the big bet at 21 a few miles outside St. Croix. On some dollar poker "slot" machines, a few passengers have won $4,000 with a royal flush after betting five coins.

There are remarkably few big losers. The one good thing about cruise ship casinos is that most of them offer no credit. You can't ask for a marker. As a result, few people lose large amounts of money because they can spend only what they bring along with them on the cruise.

But, keeping in mind the "exception-to-any-rule" law, there's at least one cruise line that offers credit. Pacquet ships accept Master Charge and Visa cards for a cash advance.