As expected, it's simply standard exposure and fill flash.
It does have an HDR appearance, and it would be interesting to know if this was just 'normal' PS enhancement / tweaking or an HDR technique was used. If HDR was used, this would be a first I imagine as an underwater photo winner.
Firstly, thanks to the staff of DivePhotoGuide and Wetpixel for selecting my image as best of show in this years competition. It is a real honour, particularly since there were so many images that could have easily made best of show. Please find some details below on how the image was captured and processed. It was the perfect afternoon for split photography. The water was like a sheet of glass and the sun was low and about to set on the horizon with hazy cloud overhead. I made several images, however this one stood out due to the striking composition. The man-o-war was almost touching the dome port when the shot was made and a single strobe was used at a downward angle to light the little critter and the seafloor (1 metre below). In post processing, I used Adobe Photoshop CS2 RAW plug-in to convert the image to a .tiff file. The .tiff was then rotated by one degree to make the water surface a straight line and slightly cropped to suit. Slight adjustments were also made to colour contrast, saturation and sharpening using Adobe Photoshop CS2.