Who are you diving with? Blue Lagoon and the Odyssey have different dive styles.
In General
1. I shoot manual and blew a lot of shots by forgetting to change settings when moving from outside the wreck to inside.
2. Really talk to your fellow divers. Nothing worse than you all rushing to one enclosed space. You may need to let eager beavers go first, keep your dive shallow and then drop down to shoot interiors after things have settled. On the Odyssey you might want to sit out the 1st dive and get on a schedule where you are going in when they are all coming out.
3. Practice your frog kick and spin kicks, it can be fairly tight in some spots.
4. I don’t do a lot of deco diving. But on this trip I became fairly comfortable with it. So know your computer.
5. I wish I had taken more natural light shots of the wrecks. I tried using the Magic Filters one dive but couldn’t get the camera to white balance, operator error.
6. Always have a focus light with you.
7. I shot the Tokina 10-17mm on 90% of the dives. Sometimes because I didn’t get a shot I wanted and went back to try again. Take your time the first round, get your shot, that will free you up to shoot some of the fantastic exteriors. There really are some great macro opportunities.
8. Try to work with the guides to find out the site schedule, if possible. That way you can come up with a plan of what your goals are to shoot.
8. So overall work with the dive guides on what the major attractions are of each site, use them to get you to the harder to find spots quickly and take your time.