You’ll spend most of your time in a tee shirt, hopefully a Dive Buddy and shorts so don’t overpack :)
Drink lots of fluids, coffee, tea and soda don’t count.
Listen to the dive briefings, I’ve seen old pro’s get into trouble because they didn’t listen to the young DM who dives this site every week.
Drink lots of fluids.
Your reg is going to be on the tank the entire cruise, don’t neglect it! Get an ear bulb and flush fresh water thru the 1st stage after every dive until the water flows out each of the drain holes. You might want to write your name with a Scripto on the bulb, others will want to borrow it :)
Drink lots of fluids.
Keep organized, anything left on the boat may become part of your tip to the crew, usually 10% BTW.
If you are going someplace exotic bring something for the village kids, crayons, in Fiji the Nai’a saved the divers used batteries for the kids. If I go back I’m taking one of those hand cranked radio/lights.
Enjoy, take lots of pictures and add any new dives you do to "Dive Sites"