A you descend beneath the ocean’s surface things become darker and bluer. Light is absorbed by the water and it does this to a greater extent at the red end of the color spectrum. When shooting in the shallows you can capture the vibrant colors of the reef, but sink a little further and your images will look a little green and possibly blurry from slow shutter speeds (or noisy from high ISOs).
The solution is to shed a little light on your subject. Send in the strobes.
I bought two used Ikelite strobes on Ebay (thank you Vadim in Australia). One is a powerful DS-161 strobe that has a 3W modeling light, and the other is a far less powerful (and much cheaper) DS-51 strobe that I use as fill. The above image is a pre-dive test in the bath, which showed that both strobes worked, and that the light from the modeling light allows the camera to autofocus on the subject. The strobes are positioned in this image for macro shooting. The lens I’m using is a Pentax DFA Macro 50mm.
Here’s a shot of me using the rig this afternoon.