Get a good
safety sausage. Bigger is better, and I like the ones that you just roll up with a Velcro wrap around vs. the ones that come in a separate bag. When you have to deploy them, having a separate bag is a good way to loose the bag! Most clip off to a dring on a BC with a plastic clip.
Dry Bag. They are expensive if you want a good one (for a plastic bag), but they are very worthwhile. A large Zip lock can be used, but IMO a dry bag is very handy when you are boat diving, and in general. If you are on a six pack, a dry bag is a must if you want anything to keep dry. On larger boats they are less necessary, but still good to organize gear.
Garbage bags to haul wet gear. Having a few large 40 gallon style bags takes up no room or weight, but is invaluable if you have wet gear.
TSA locks. This is hardly a must, but I like the ones that tell you if you luggage has been searched.
Zip Ties. These handy lightweight THINGS are invaluable if you need to attach part A to part B!
Defog - You can certainly buy this in Coz, and likely other divers will share, but it's a must have unless you like using spit which some divers use.
Sink the Stink - another item you can purchase in Coz. I like to destink my wetsuit usually a couple times during a week of diving. For rentals this actually maybe MORE important as they tend to get riper quickly!
Silicon lube for camera orings. You should be keeping your Oring in good shape. I lube mine daily when diving.
I dove with a knife attached to my BC last January in Coz, but you are not suppose to have them. I never removed mine from the sheath. Gloves are also a no no in Coz. These are marine park rules, so don't be upset with the DM who tells you to leave knives or gloves on the boat.