Hi Yaesumofo! :wave:
Yeah, I thought about picking up one of the small color checkers to do just what you suggested but candidly, with PhotoShop CS4 I am satisfied with tweaking the color temp and tint in "Camera Raw".
A white card under water is not white anyway. It is typically slightly green or slightly blue/ cyan depending on the depth and water clarity. Unless you are shooting macro, you typically are using a very wide angle lens if not full on fisheye lens. You have already distorted the image relative to what the naked eye sees. When I used to shoot film, the lab that did the processing had their own adjustments they would do to partially compensate for the heavy blue and lack of red and the results were reasonable but not close to what you actually saw, color wise. Now that I am shooting digital and have software that allows me to do my own adjustments and indeed enhancments, that is what I attempt to do. In most cases, I try to get the colors to represent what I saw but in some cases I will attempt to get the colors to what I would have seen had the conditions been pristine. :shrug:
By experimenting with some of the images that do contain a white surface (underside of a ray's wing or the white on a humpback whale) I now have a sense of the corrections required but to render these white surfaces white, in most cases, you dial in so much red that you get some red flares and artifacts elsewhere in the image which detract from the image on one hand and they certainly were not seen at the time the pic was taken.
Historically, the professional UW photographers would bring their own light sources to the shoot and flood the scene as much as possible with artificial, full spectrum light. Stunning images resulted but images you never saw in person!! Red is a common color adapted by much of the underwater creatures because it is a color that is not present in their habitat and as I understand it, many under water creatures don't even have vision in the reds because it is not light encountered down there.
Here is perhaps a good example of color rendering and how it might fit in. I missed out because nobody told me but last year, there was a rare Hawaiian Sea Horse residing outside of the reef I usually swim. It was down at ~ 30' so I would have needed to use scuba to visit it. At any rate, in the weed field it hung out in, it appeared to the naked eye as the same dark and dull color as the weeds. In reality, it was a really pretty red! It was first spotted by divers who had some dive lights with them. With a bright dive light, the Sea Horse would pop right out of the green/brown seaweed. A diver I see at the beach all the time who takes tourists out on dives showed me some of his pictures and I gave him some grief for not telling me about the Sea Horse. In his defense, at the time it was there, He and i would just nod heads at each other as we had never had any dialog.
So I guess the point is that without artificial light of adequate intensity (in the reds), one was likely to miss seeing this fish to start with. If you did happen upon it, would you want a photo of it as it appeared to the naked eye which was a dark non colored animal or would you want a shot showing its bright red as it would appear, say at the surface? I would think the latter.