Ben,
Of the many times that I have looked my Nikonos in the face, I never really noticed those threads! However, they are there and they will take a standard 58mm filter.
I can not remember any article or book that ever mentioned using filters u/w. And I just checked my cc of Wu, Edge and Church: not one mention of filters.
So, I used my imagination (very dangerous) and except for some photography at very shallow depths and, maybe, for some artistic effect, I cannot see a use for them. I guess in a shallow, natural light situation you could use an orange or red filter to compensate for color absorption. But, since the filter would require at least one f stop larger opening to compensate for light loss it does not sound too promising.
If you were going deep, a strobe will, generally give you the white light you need and no filtration would be necessary. Further, since the absorption would vary with depth, you would not get a uniform effect.
Finally, my guess is that, if you chose to use a filter, you would be well advised to put it on underwater. There would probably be leakage by the threads, but it would be better to have hydrostatic pressures as opposed to compressive presures on the filter's glass. I just do not know why you would need a filter.
With the exception of a UV filters, polarizing filters, and some soft focus filters, most color work is not done with filters. Black and White photography makes great use of filters, the most popular being red to accentuate clouds against a blue sky, for example.
All that said, it is possible that there would be artistic value in using a filter...and as sure as shooting...if I tell you it isn't worth doing, someone will pop up with a Pulitzer Prize winning photographer who uses filters regularly u/w!
I think that I will send off an e-mail to Nikon and see what they say!
Joewr...
