Original question was pertaining to Olympus C7070 camera. Following explanation is from our knowledgeable types:
That is a C7070 glitch that we talked about before. The 7070 will do the exact same thing with the Olympus land flash units. The solution is to open up the aperture until his pictures are not dark any more.
The reason his system did not do it as much on land is because he was not out of range of the flash as often in the room he tested in. Once he takes the system underwater, the point of underexposure is more easily reached due to the attenuation of light by seawater.
The 7070 will not slightly underexpose any images, they will be either correct or severely underexposed. Once you get beyond the range of the flash, the camera sends data to shut the flash off very quickly.