I recently replied to a question about etiquette in the main dining room on "the other boards" with a twelve-point synopsis that got a far more enthusiastic positive reaction than I ever expected. On the off chance that it might be helpful to somebody, here it is with some minor editing related to the absence of context of the original question.
Dining Room Etiquette
>> 1. Come to the dining room on time, wearing the attire prescribed by the host (the master of the vessel, through the daily program) each evening. It's quite proper for members of the armed forces to wear the uniform that's equivalent to the prescribed attire. (On "formal" evenings, the proper uniform is the so-called "mess dress" or "dinner dress jacket" uniform or, for members not required to own that uniform, the substitute prescribed in the service's uniform regulations.) Cultural variations such as the Scotsman's dinner jacket worn with a kilt and the Bermudan's tuxedo worn with formal shorts (with the same satin stripe as standard formal trowsers) and black knee stockings on formal evenings, are also appropriate, so long as they are socially equivalent to the prescribed attire. If you make other plans and you are not coming to your assigned table in the dining room, communicate this fact in advance to your waiter, and, if possible, to your tablemates.
>> 2. When couples arrive at the table, the lady should sit to the right of the gentleman because.... well, because a gentleman always shows respect for a lady by giving the place of honor, which is to his right, to her. The only exception is if the gentleman holds an office of higher rank, which would entitle him to greater honor as a matter of protocol, in which case it's acceptable either to follow protocol or to follow the normal social custom.
>> 3. Always greet your tablemates and your waitstaff with a smile and a chearful "Good evening!" This gets the evening off to a good start! Also, be sure to complement the ladies at the table on their appearance -- especially on "formal" evenings! Try to keep the conversation pleasant and cheerful. If somebody says something that you find disagreeable, you can smile and say something like, "I really don't agree, but let's not spoil the evening by arguing about it..." and segway to a completely different subject as cheerfully and smoothly as you possibly can. (It's best to avoid discussion of the three taboos -- women, politics, and religion.)
>> 4. As soon as you sit down, open your serviette ("napkin") so it's folded in half and put it on your lap. Don't wait for the waiter to pick it up and hand it to you. If you leave the table before the end of the meal for any reason, place your serviette on the seat of your chair -- NOT on the table -- until you return.
>> 5. Decide what you are going to order as soon as your waiter brings your menu so that you are ready, and thus don't hold up the table, when the waiter comes to take your order. (Tip: Most ships post the menu for dinner right outside the dining room during the day, so you can review the menu and make your selections in advance. That way, you don't have to divide your attention between the conversation at the table and your menu.)
>> 6. Most cruise lines set each place with the forks to the left of the main plate and knives and spoons to the right, in the reverse of the order needed, up to and including the main course. Thus, it's proper to use the outermost utinsils first. If you skip a course, skip the corresponding utinsils if the waiter fails to remove them. The untinsils "above" the main plate are for courses after the main course -- that is, for coffee or tea and dessert. A few cruise lines set the table only with utinsils for the first course and bring the utinsils for subsequent courses with the respective courses, so "which utinsil to use" is not an issue.
>> 7. The bread and butter plate is always to the left of the forks and the beverages are always "above" the knives and spoons. The bread and butter plate seems to be the one that somebody invariably messes up, whereupon several people to the right of the initial offender follow suit because they don't want to embarrass the initial offender. Thus, it propagates around the table to the other side where somebody gets caught between those who get it correct and those who get it wrong, and has the choice of making a scene or doing without....
>> 8. Wait until the waiter finishes serving each course before picking up your utinsils to eat. If some passengers skip a preliminary course, the waiter may serve their next preliminary course (but never the main course) to them while the others are eating the courses before it to achieve better synchronization of the whole meal. In such situations, the rule applies to those who are in the same situation rather than to the whole table.
>> 9. If somebody else at the table shares a bottle of wine and you accept, reciprocate when the bottle runs out. If there are several people who accepted the initial offer, share with all of them.
>> 10. If you have any sort of difficulty with your tablemates or your waitstaff, be civil for the rest of the evening, then see the maitre d' as discretely as possible and ask him (or her) move you to another table for the next evening. (On some cruise lines, you can call the maitre di' during the day to request a change of table for that evening.)
>> 11. If your tablemates decide to go to an alternative restaurant as a gruup, go along. You just might enjoy the change of pace.
>> 12. Above all else, be polite to your waitstaff. Simple courtesies like "please" and "thank you" and addressing them by name, rather than "hey, you!," mean a lot, and invariably motivate them to go the extra mile for you. On the last evening, make sure that your waitstaff get the proper gratuities, calculated based on the length of the cruise rather than the number of evenings that you actually ate in the dining room. They are there for you even if you eat elsewhere, and they do a great job!
I realized, in the discussion that ensued on "the other boards," that this synopsis neglected a few points like sitting erect with one's tush at the back of the chair, keeping one's elbelows off of the table, cutting one's food one modest-size bite at a time and eating that bite before cutting the next, breaking one's bread into bite-sized pieces and buttering one piece at a time, spooning one's soup properly, etc., which obviously still apply.