Q.My husband and I recently purchased a boat, which we keep at a local marina. We are busy people, and what little time we have together frequently finds us sailing.

We have, in the past, invited friends to accompany us, most of whom are lovely people whose company we dearly enjoy. My problem is how to deal with a few people who insist upon honoring us with their presence whenever they feel like it, giving no thought whatsoever to the fact that we may already have plans, or perhaps we simply desire to be alone.

I entertained the thought of running to the bow, screeching loudly, "How did you get in here? I don't remember inviting you." However, screeching hurts one's throat, and the idea was immediately dismissed as being too crude. Effective, probably, but crude.

It would be wonderful if you would suggest a polite but firm way for me to deal with my unannounced, uninvited and unwelcome visitors.

A. Have you tried casting off. When you see them coming and then waving cheerfully as you go out to sea? It would seem to Miss Manners that unwilling potential hosts on a boat have a decided advantage over people caught at home in the same circumstances.

Nevertheless, everyone, everywhere, has to know how to escape being at the mercy of unannounced, uninvited and unwanted guests. In summertime, especially, many people make the rude assumption that anyone with a desirable recreational facility is obligated to run a public accommodation. Ask any owner of a swimming pool or a vacation house.

It is always possible to say no politely; the trick is not to offer a plausible excuse. Oddly enough, it is the excuse that offends people, even if it is not, as is often the case, a stumblingly delivered lie. "We feel like being by ourselves today," which is truthful, is worse than nothing.

So instead of screeching, just say politely, even sympathetically, "Oh, dear, I'm afraid we can't take you out on the boat today. What a shame you came all the way out here for nothing."

If they press you, with an incredibly rude "Why not?" the most you can offer in the way of an explanation is, "Well, you see, we made other plans. We're so sorry."

Then cast off.

Q. I have a question about balls.

Does a girl of 16 need a date for a (black tie) ball? Can she go alone with her family or a friend? Also, could you explain how to present a debutante? How old are debutantes?

A. Debutantes are supposed to be 18 and innocent, but nobody, especially not Miss Manners, checks them at the door.

As the purpose of a debut was to present a girl just coming of age to adult society, the event at which it was done was given by her parents or other close adult relatives. That was generally a tea, or a small home or club dance, in addition to which several families might combine to give their daughters a ball.

Miss Manners speaks of this in the past tense because the mass events, run by hired specialists, in which a girl who has been running freely since she was 12 is "presented" to people her parents never saw before, often in a city they don't even live in, bears little relationship to a debut.

However, as there is no purpose left for the old-fashioned event, designed for a society that believed in keeping its daughters secluded, Miss Manners has no objection to a modern young lady's going out dancing and calling it whatever she wishes. If it is a multigenerational dance, as opposed to a prom, which people attend in couples, she may certainly go with her parents.

Commercially run balls often require that the young women have one or several "escorts," in order to provide extra dancing partners, or to populate those sideline groups of young men who drink too much and make unpleasant comments about the debutantes.