Underwater Photography - Attachable Flash

To avoid backscatter, I am considering purchasing an attachable flash for my Fantasea underwater case which fits my Nikon S600 compact digital camera. It appears that the attachable flash is triggered by the built-in flash via a fiber optic cable and sensor placed in front of the built-in flash, therefore both will flash when I take under water pictures. I do have a black-out for the built in flash, but it doesnt block all the light so a fair amount of light will still be omitted. Is this still likely to cause backscatter? Do all attachable flashes work like this?

Although the flash from the strobe will be much brighter than the internal flash of the camera, I just cover the part of the housing directly in front of the internal flash with duct tape to be certain. Any light from the internal flash that escapes upward or to the sides through the housing will be going in the wrong direction to produce back scatter.