Simple underwater lighting
1 Trigger the flash
You need to go old-school and use the camera’s pop-up flash to trigger the two remote flashguns (each set to optical slave). For simple ‘cross-lighting’, position one flashgun behind and above the water, and the other to the side of the camera with a diffuser panel to soften it.
2 Backlight the body
Backlighting the subject with a flash from behind creates an edge highlight along their side, separating them from the background. Once the flashes are in the bags it’s hard to adjust the power, but you can move them closer or further away to control the intensity.