I was there in July - thanks to Valdi and Fraser for their tips and assistance.
If you are able to dive Silfra daily, you have a good possibility of finding the optimum conditions:
- a sunny day. Try different times of day to see when you get the most dramatic scenery that also shows distance in the clear water behind
- You might want to try shooting angles where you place yourself close to the edge of a sunlit/shadow cast area.
- Try shooting very close to the surface on a day with no wind to get either a mirrorlike or slightly rippled reflection of the scenery
- You need a model (one or two) in the scenery to give scale. Perhaps one fairly close and anotherone siloutted further away, in a nice position.
- Try to emphasize the airlike transparency of the water in Silfra, .e.g by showing empty water below the diver so that the diver seems to float in air.
- Try to utilize how the color of the rock seems to change beautifully from orange to blue. i.e. consider having some rock close to the camera. For example a scene from some shallow or narrow part towards the deeper blue views.
- You probably will not benefit much of strobes. However, the mass/inertia of the strobes and arms will help you stabilize your camera when shooting in ambient light. You could consider a tripod as well.
In addition to Silfra, I am sure you are aware of the many other water-filled cracks in Iceland...