Depends on if you are doing wide angle or macro or fish portraits.
Generally put the camera in full manual, use TTL if you have that option, set white balance to something like cloudy (works good on Nikon anyhow), set your focus to center and exposure to center weighted. You don't want the matrix focusing if you have that option to be trying to focus on several things. You probably won't be able to use super fast shutter speeds with your camera because of the sync speed. It may be as low as 1/160. Mine is 1/250 on my Nikon. You'll have to play with it on land to find your max or look in the book
For wide angle, you'll use a slower shutter (1/60 to 1/125) and a more open aperture (F/7 to F12). You'll use your meter in your camera and meter your blue to be about -2/3 under exposed using your aperture by pointing it out in the water first, then the shutter speed is used only to adjust for speed. It can be left alone most of the time. If you are attempting to get a sunball in your picture, you'll want to use the fastest shutter speed and a smaller aperture to capture the sun ball and light rays and not over expose the sunball too much while keeping that nice dark blue.
Macro is quite a bit different, here you want to use your fastest shutter speed most of the time that you can with the strobe. The aperture will be shut down (higher numbers) in the range of F16 to F22 or more depending on the lens but basically you are trying to limit the amount of light entering the camera so you can put light on your subject but blacken out the background. The faster shutter keeps the picture crisp even with moving subjects.
Fish portraits with macro lens, you'll just need to slow the shutter down a little and open the aperture up like you were going to take a wide angle unless you are trying to take a close up of the fish then go back to macro settings.
The main thing to remember, don't forget to get close and fill the frame with your shot. With my macro lens, I am so close at times that I have to be careful that the lens doesn't actually touch the subject. Wide angle even, I get VERY close. My 10.5mm lens I can get within inches of the subject. Use the strobe to the side for edge lighting, don't put it directly behind you or you'll get back scatter.
Since this is a new rig. Take LOTS of pictures and change settings and strobe positions so you have choices when you get back. You can then look at that day's diving pics and see what works and what doesn't work. Then you can make adjustments the next day.
Have fun!
