Regarding white balancing, which I am looking forward to be able to do underwater with the CX550V (not so with my previous camera and housing), I practiced a bit above water (as recommended by Peter) and it appears that what matters is not the actual color (white) but the fact that the color filling the frame has equal RGB values.
In other words, any shade of gray (that is not too dark though) does the trick. I tested that with a black and white (and gray!) poster attached to my white wall. The camera electronics appears to correct each color pixel value such that for the actual scene, the result is an image with equal R, G and B value. The fact that it works with any shade of gray seems to indicate that what the camera does is a simple rescaling of the raw pixel value.
To illustrate what I mean, suppose that you are shooting a gray slate at the surface, using a "white" light. A perfect camera (which I'll suppose the CX550V is) will see a scene comprised of color pixels reading the same value, no matter what color filter they have in front of them, i.e R=G=B= say, X).
[Oh yeah, each "pixel" of an image is in fact the result of reading 4 sub-pixel values, one red, one blue and two green, in general...]
If we take this slate underwater at depth, there is much less red transmission, so our camera will read for instance G=B=X, but only R=X/2. White balancing simply tells the camera: when you read R = X/2, report R = X instead. The value of X used to measure the respective values of R, G and B does not matter for that purpose. What matters is that the target has R=G=B (i.e. is gray) IF SHOT IN WHITE LIGHT AND NO ABSORPTION BY THE ENVIRONMENT. And also of course that we use the same illumination light than when we will shoot the actual scene.
A corollary of this is that if you white balance with an object which has say, a larger R value than G=B value under white light (a red curtain for instance), the camera result will look blueish/greenish, because it will artificially reduce the red pixel values (I confirmed this too). Experimentally too, it seems that you don't need to fill your frame with the white or gray object, as long as it covers a significant part of the scene. For instance, I managed to white balance correctly filling more than 75% of the field of view with my red curtain and having the rest filled with the white wall. The camera electronics apparently looks for a bright cluster in the color histogram and decides that this is what he has to call "white/gray". So you might not need to zoom fully on your fins or slate as long as you have it on a dark background (so that the camera is not fooled in believing that the background is actually what it has to interpret as "white/gray".
In summary, if you are afraid to be ridiculous with white fins, you could as well use light-gray fins! And better yet, if you happen to have one handy, you can white balance on a Great White or any gray shark/fish swimming nearby... :-)
