PEOPLE WHO go cruising on ships worry a great deal about the niceties of etiquette. Actually, it is the people who do their cruising in shopping mall parking lots who should be worrying about their manners, in Miss Manners' opinion. Shipboard behavior is comparatively simple.

The principal activities in which passengers engage on cruise ships are: eating, dressing, tipping and complaining. You will notice a similarity to pastimes in which many people engage on dry land. The difference is that on shipboard, you do a great deal more of each. There are also other options, such as seasickness and buying baskets in port (or port in baskets), but these every-person-for-himself basis.

Miss Manners will assume you have mastered the basics of each main activity at home, and will explain only the differences you may encounter at sea.

You have been taught to be friendly to whatever companions you find at the dinner table, to eat everything you are offered and to keep all foods confined to plates, the bowls or prongs of eating utensils or the mouth. None of these rules quite applies on a ship.

Ships serve the following meals: breakfast, elevenses, luncheon, tea, dinner and midnight supper. Most people attempt to eat them all, including all the many courses of each, on the logical grounds that they have paid for this access to unlimited amounts of food. This is a mistake. Many people find out the hard way just how bad a mistake it is. Take Miss Manners' word for it.

When you consider how time-consuming it is to consume even some of this food, you will realize the importance of having compatible table companions. This is why the first thing you do aboard a ship is to get a table reservation, either alone, with the people with whom you are traveling or in a table to be arranged at the judgement of the dining-room steward. If you choose the latter, the second thing you do aboard ship is to ask to be moved to a less incompatible table place. This type of discreet snubbing may be applied to anyone on the ship except its captain. If you have the tremendous honor of being asked to sit at his table and can't quite bear his conversation, it is necessary to plead seasickness and to re-assign yourself a table in your own cabin.

Spilling food is permissible aboard ship, if accompanied by a dark look over one's shoulder, to implicate the stabilizers and the ocean itself in this transgression.

Dressing on cruise ships has become Standard Tourist, which is to say sports clothes during the day and restaurant clothes at night. Miss Manners deeply regrets the general passing of wearing black tie to dinner and applauds those few who steadfastly stick to the custom. Of course, one does not so dress the first night out, the last night out, the night before getting into port, nights when the ship is in port or nights the ship is leaving port. On most cruises, this accounts for every night there is but Miss Manners would still like to see evidence of the intention, such as a steamer trunk full of evening clothes in case the ship is unexpectedly stranded at sea.

Tipping, which most people fear, is actually easy, as all ship personnel are willing to tell you what is expected, and then some. Five percent of one's fare, which is what they recommend, is generous. Envelopes once a week, with $5 to the waiter and a dollar or two less for cabin attendants is a minimum. Wine stewards get 15 percent of the wine bill, and bartenders 16 percent of the bar bill. Others who perform special services are tipped as they would be in hotels.

As for complaining, it is the general cost of cruising that inspires many people who are otherwise meek and patient with whatever life gives them to become highly critical of everything the ship has to offer.

As cruising should be vacation time, Miss Manners recommends this constant harping only to those who find it relaxing. Personally, she prefers to sit in a deck chair and read "Moby Dick." MISS MANNERS RESPONDS

Q: My lover and I, who do not live together, entertained a friend and his lover, who do not live together, at my place. Since then, both guests have called independently of each other to thank me. They were aware that dinner was a joint effort on the parts of my lover and me but he has not heard from either of the guests. What are we to make ot it all? One other point: It was my lover who had originally introduced us all to one another, so he was everyone's friend at one time.

A: Are you suggesting that four telephone calls would have been proper after one dinner -- once call from each guest to each host? Miss Manners has enough trouble trying to get one such courtesy out of anyone. It is fair to assume that you and your lover, being lovers, are in touch with each other, and therefore that you would communicate the message of thanks to him. One of that pair could have performed this task for both of them. Isn't that what love is all about?