Hi waterdogs,
The truly impressive list of posts above well covers the many tangential issues relating to the harmonious mix of newbies & old timers. Now for the core items:
1. If the boat has a marine head, learn how to use it so as not to fatally foul the facilities. It don't work like the toidy at home unless you live on an old houseboat.
2. If you suffer mal de mer, go far forward or aft & hurl well over the rail. Under no circumstances direct your rainbow toward rinse buckets, trash containers, marine heads or the beverage/snack cooler.
3. Related to #2 above, it's often a good idea to skip the habanero pepper, scrapple, Vegemite & dated sour cream omelet at breakfast. Ditto the SPAM & olive loaf on dark rye with heavy mayo washed down with blueberry Gatorade over by the diesel exhaust vents during surface interval.
4. If you must wet in your thermal protection, and this is to be expected, do so while submerged. Trying to slip one by while suited up onboard will fail the visual & olfactory tests unless you're wearing a dry suit.
5. Mastering buoyancy control can wait. First practice, practice, practice putting your first stage on the cylinder. If you are observed having to reorient it, you're a terminally marked diver. Ditto for removing the cylinder restraining bungie before rising. Being thrown to the deck as a result of such failure can be hard on both the ego and the coccyx.
6. Also practice your basic hand signals. Giving a "thumb up" to indicate you are doing swell & having a blast not only can cause serious confusion, but means you cannot pass go & cannot collect $200 in the "Scuba-Opoly" game.
7. If you are so foolish as to wear sunglasses without a croaky, hat without a snugging band or T-shirt/towel draped loosely over the shoulder while the boat is in motion and one of these go overboard, just eat the loss quietly. Don't scream out, "Stop, my....."
8. Carry recommended safety signaling devices, but prepare & attach them properly, and keep your cotton-pickin' paws off 'em unless needed. A streaming safety sausage is an annoying sign of a newbie. And fingers away from the Dive Alert. Even the many hard-of-hearing among the "experienced" divers find that a toot a few inches from the ear disturbs their solitude.
9. Rinse your nose & mouth before climbing the ladder for reentry to the boat. Reentry sporting snagged snot or a lengthy lunger is considered bad form by some.
10. If you are a male, don't become distressed if you experience some degree of "shrinkage" due to cold water and thus present a less than incredibly virile image in your wetsuit. Trying to compensate for this by using a spare dive sock or octo retainer is not likely to fool the old hands for long.
But, seriously, as NetDoc said, the fact that you "are even cognizant of the older divers on the boat" means that you are ahead of the game.
Just remember what you've been taught, and be careful, thorough, alert, and respectful of others & the marine environment.
Finally, don't be intimidated. I can only joke about the above because I'm guilty of committing them. Well, all but #10.
Have fun.
DocVikingo