Mark,
If you take a strobe like the suggested YS-110, it has a guide number of about 15 in the water, so if your camera is set at f5.6, the light will only perfectly illuminate about 3m away (you divide the GN by the aperture). So really don't think of a strobe as being able to illuminate further than a couple of metres away. So in a wide-angle scene, you need something close to illuminate:
So don't think of the strobe as the answer to everything, it can't perform miracles. So what you find is your kit dictates what you take, not necessarily the other way around.
Rob
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East Midlands Underwater Photographers www.emup.org.uk www.robcuss.co.uk
Camera kit: Nikon D80 in Ikelite housing, Tokina 10-17mm, Sigma 50mm, Sigma 105mm, twin Ikelite DS-125 strobes