Of course there are the obvious ones such as: no saving seats, arrive on time for dinner in the MDR, don't slam cabin doors, hands out of the buffet trays, waiting for occupants to exit the elevator before attempting to enter, etc.
I'll start by generating a few suggested etiquette expectations of cruising:
1. Don't complain while waiting in line for tenders, meals, etc. When people are on vacation, they don't want to hear your grumbling/complaining about what isn't right or what cruise line does a better job than Carnival at whatever. This is a downer.
2.Don't cut in line or save a spot for folks, who then show up and there are ten in the party. Asking the person directly behind you if they mind doesn't count. What if the next person minds?
3.If you order a bottle of wine, consider sharing with the other diners. (I know some will disagree with this, but to me this is just good etiquette.)
4. When entering an elevator that is darn near packed, please cease your conversation upon entering and resume it when you exit. Everybody doesn't want to hear what you have to say.
5. If you are on the Lido, where alot of folks are circling with their trays trying to find a spot to sit and eat, please consider vacating your spot if you are playing cards, dominoes or reading a book. There are lots of other areas for activities such as this.