First thing I wold say is give yourself time. So,
1. Set up your set whilst in the dock before leaving harbour if you can. - Less likely to get seasick, less likely something will fall if the sea is a bit lumpy.
2. Check your set is tied in and secure, if the sea is rough check it regularly.
3. Put all the bits and bobs you need for the dive in a string bag, (I normally place this under the seat or attached to the set. (Mask, Fins, Gloves, Hood, Reel, DSMB) i.e. just what you need, all in the same place (not scattered around the boat). Place weight belts in the weight belt box (if they have one) or under the bench (- if they don't).
4. My computer is normally on my wrist or clipped to the D Ring on the set.
5. Put large bags or crates out of the way of the benches, entry point and exit point.
6. Know when you are diving. Then you can change in good time and be ready.
7. Confirm who you are buddied with - tell them your level of experience, what you want to achieve, your weaknesses or concerns.
8. If they are organised, find out what order they are putting the divers in the water. - I would suggest you are towards the back, less rush fewer bodies in your way.
9. Let the Boat Dive Master know you might want a little help with the kitting up - if you feel this will help you.
10. If you return to the boat early, with a lot of gas, you can always do 'little excursions' to and from the boat as you use up gas - don't push it. If you are with a DM, you could always do a few 'skills' - but make sure this is in the buddy briefing before the dive.
ENJOY THE DIVE - which also means not trying to achieve to much.
11. Once back on the boat, secure your set!
12. If the practice is remove the reg' for a fill DO THIS!
13. Gather all your bits and bobs up, place them in the string bag with your kit. (Best NOT on a bench - someone sitting down with a set on or moving a cylinder might land on your nice new mask!)
Place weight belts in the weight belt box (if they have one) or under the bench (- if they don't).
14. GET OUT OF THE WAY of the entry/exit point, ladders and benches.
Have a chat with your buddy, and the other divers by all means, but not in the way of divers getting out of the water, or the crew trying to fill cylinders.
Also know where the boat dry areas are (and wet areas), don't make yourself unpopular walking into the dry area in your wetsuit!.
Also - if you don't know the practices on the boat, or why, or why not, ask.
Most importantly ENJOY.
Gareth