herman:I was not thinking of not enough light but rather lighting angles. The ext strobe will be coming down from above-at least with the present mount I have, an Inon DIII and I was thinking the head on lighting from the internal strobe may improve/reduce the shadows when used in combination with the external strobe.
I see what you're gettin' at. If you place your strobe directly over your camera, in fact mine touches the top of my housing, aimed at your subject you won't have severe shadows from above. For really close macro I lower the power of my strobe a bit to prevent blowout on the light areas and to deminish shadows. The light is coming from the front and slightly above at an angle. If there's any particulate in the water, the angle of the strobe will prevent the backscatter from being highlighted. Whereas the light from the internal flash goes straight out and bounces off the particulates straight back into the camera.
The only way to be sure is try it!