The main problem encountered so far has been with back scatter caused by light being reflected of suspended matter in the water.
Mild cases can be salvaged by touching up with photo editing software.
Back scatter is not so apparent when taking photo's of light subjects against light coloured backgrounds (see below).
Note how back scatter is far less apparent on the lower half of the image.
I had expected the problem in low vis. the top two images were taken in about 3-4m vis. but I had expected it to be far less in clearer water.
Unfortunately this was not the case, the image above and the one below were both taken in 15m vis. on the Kintuck.
Photo's taken with ambient light do not suffer from back scatter but you are limited in the type of photograph you can take and the depth you can take photographs at is also limited due to light absorption, which also leads to a loss of colour due to differential absorption of different wavelength of light*.
Photo taken in 4-5m vis using ambient light, depth about 15m, overcast.
This image was taken on the same dive with the internal flash!
So, is back scatter a serious problem?
Yes, if you want to take photographs of subjects that are more than about 0.6m away, but if your happy with closeups and "atmospheric" non flash images then I think you'll be happy with the results.
Is there a way around back scatter?
The books say that that using an external flash will improve the situation but I can't quantify the improvement until I get one. I think that will have to be my next photographic purchase, but not this year!
* This can be improved by adjusting the colour balance and adding red or by using a coloured filter (which I have no experience of).