Thanks for the reply!
I did some tests white balancing on land (in my dark basement) but was not pleased with the results... Perhaps I did not use a good grey card?
Well, it seemed OK on land, but my guess is that the water filtered the colors from the lights underwater so the white balance in air was not relevant.
I remember reading about WhiBal long time ago but it never struck me that I could use it underwater.
It seems a bit small though!?
Is it really possible to use it with the Tokina 10-17 mm fisheye (or Nikon 10.5mm fisheye)??
Also, I guess it is less reflective than my "white board", but how reflective is it (very, quite a bit, not so much , almost not at all ;-) ...)?
If you are concerned by the size, they have different sizes, I haven't tested it with my Tokina 10-17, but I can do some test during the weekend.
Concerning how the card is going to reflect the colors, I'm sure that the grey card will perform better than your PVC card.
I don't have the technical knowledge in order to explain why the grey card works better, if you go to the Whibal
site you will find some useful information about white balancing.
The only thing I know is that the card must be as neutral as possible, you can have a neutral color with white, black or grey.
The problem is that is too difficult to produce a true white or true black, but it's easier to produce a neutral grey card.
Not only the color of the card is relevant but also the material, the material of the card must reflect the colors of the light in the same way independent of the colors present in the light that you are using in a specific moment, and for us the material of the card must be waterproof also.