Jim Baldwin:
Get a good marker and put your name, initals, outline the letters or something on your gear so that you can determine what is yours. This is especially important with a lot of folks on board. You will stand a better chance of getting your fins, mask, and weight belt back in its right place with your mark on it.
And as several have said be sure and take plenty of seasick medication. Start taking it the evening before so it gets in your system. Bonine works great for me and doesn't work for others. Stay away from the back of the boat. The heavy diesel smoke will do most people in and don't put your wetsuit on at the dock. The boat captain or first mate will normally tell you when you are about 10 to 15 minutes from the dive site usually during the dive briefing. This is plenty of time to get into your wetsuit and will keep you from overheating.
I was going to mention this point. We are new at this too, and I was very concerned about know how to 'be' on a dive boat. You have gotten great advice, and I was going to bring up this point...
I will preface this with the fact that I'm a total noob.....but while my experience is limited, I've been through my first boat dive within the past month. I was sort of hyper-sensitive to some things, so hopefully this will help you...
Re: overheating in your wetsuit. When you come up from your first dive (in a 2 or 3-tank dive), you can ask the captain about how long before the next dive. If it's any length of time at all, I will unzip my wetsuit and at least get my arms out (tied loosely around my waist) so I don't get too hot. You'll usually get another heads-up when you are nearing the next dive site so you can re-zip.
They will usually stress this during the pre-trip speech... If the tanks are secured with bungee cords, make sure to re-secure them once you are finished getting set up...and any time you are not actively working with them. Also, remember to take the bungee off before hooking your regulator and BC to your tank...otherwise, your gear will be attached to the boat.
Before going in, make sure you know what you are looking at. Hopefully, the captain or someone will brief you on your location and what to look for. He will hopefully tell you where the reef (or whatever) is located and about how long you have underwater. If you have any questions about where to go, ask before you get in. Once you get in, be sure and signal to the captain/crew that you are OK (big signal on head) and then get out of the way of the next diver.
Once you are in the water, if you feel you do not have enough weight, you can sometimes borrow some from the boat. Just be sure to return it before the end of the trip. They usually have at least some weight clips available, but try to have what you think you'll need. If this is your first salt water dive, don't forget to account for that...
Not sure how much they stressed this to you in class, but remember to not be behind another diver who is going up the ladder. If they lose their balance and fall back, you don't want to be there. Also, when you go up the ladder, you might want to keep your reg in your mouth in case you fall back into the water...so you can breathe with no probs...
Unless they tell you otherwise, when you come up you will hand (not toss) your fins to the crew before going up the ladder. Some people would prefer to put their fins on their arm and not hand off their gear. As soon as you get up from one dive, switch tanks (if applicable) and get ready for the next dive.
Most places have water or other drinks available for everyone...sometimes snacks (pretzels/cookies). Pay attention to what the captain says is for everyone. Sometimes, other passengers bring along their own water/drinks, and you don't want to go in the cooler and take someone else's stuff. If you know you can bring your own stuff, don't do glass containers.
This is something I'm sure you wouldn't ever do (considering your effort to know dive boat customs), but I couldn't believe this guy on our boat last week. I thought the captain was going to pitch the man's cell phone out in the ocean -- and I would have understood (and applauded). He talked from the minute he got out of the water until we returned to the dock. It was annoying and pointless, and it really took away from the relaxation of the boatride back....listening to this guy 'talk shop' while we were on vacation...
Like I said, I certainly don't have much experience, but I have done this for the first time - very recently....it's still fresh on my mind... I was really nervous at first, but everyone was really great and we had a fantastic time. Just be really aware of yourself and others.