This is one of the reasons I have argued in the past with those that say the MMII is just as good as a NikV. ReyeR is right; it is caused by backscatter. The Square effect is from the poor (read cheap) design of the MMIIs aperture diaphragm blades. The reason you dont see this on land is that there is normally no particles floating around, between you and your subject that is being lit up by your flash. I bet if you took a similar photo in the rain or with snow falling, it would appear similar. The reason for the squared look, is that the aperture only has 4 blades. You can see this by looking into the front of the lens and moving the aperture or F-Stop control. Higher quality lenses have 6 or more blades. This makes the aperture opening more round. Because of the MMIIs or MMII EXs square opening, all light flares or out of focus highlights (scatter) will appear as square ghosts. The size of the opening will affect the size and sharpness of the ghost. The smaller the opening, (higher F-stop number), the smaller (and sharper) the ghost. Highlights and flare is not the only thing that is affected by the square. You will also notice that some out of focus areas will appear squared, stepped, digitized, or pixilated. The 4-blade aperture also causes this. This is why better made cameras dont use this cheap design. Can you work around this (flaw)? Yes, the recommendations by ReyeR of Strobe positioning and reducing the water column (and hence the number of particles) will help you reduce this effect. Also, burn a lot of film. Film is the cheapest part of your system; dont be afraid to use it. Off soap box..
