Hi,
I suspect that the absorption of energy may be more dominant that scattering. absorption would be more like the molecules of water consuming some energy into vibrating molecular bonds. Scattering would be more like a re-direction of light so that it does not get to you down below.
As for your experiment, I think that one of the key things to make sure you do before you go down, is to understand what you are going to use for a response. Since the camera may try and adjust lighting conditions, you could cannot rely on it as an "absolute" reference, meaning if you were to photograph a single red card by itself at various depths, you don't know how the camera is going to adjust exposure time on it's own. Instead, you could use a metric which compares the intensity of a red sample to a blue sample within the SAME photograph, at various depths, and take a percentage of one from the other. You could use the RGB values in photoshop for this. For example, here is some made up data:
Depth Red Value Green Value Blue Value Red as % of blue Red as % of green
0 225 235 235 =225/235 =235/235
30
60
90 20 50 200 =20/200 =50/200
This gets you half way there. To take the experiment further (which may not be necessary), you would want to also construct the same table in similar daylight lighting, out of the water, but use varying camera exposures (instead of depth) to make sure that the relationship between red, green, and blue don't change dramatically with just lighting intensity. With that, you can get a complete picture of how the colors on you color card are changing RELATIVE to each other. Again, an absolute measurement with your setup may be difficult. Go for relative. A final note is that if you can control your camera shutter speed and aperture, it may help to keep this all constant so you don't have to consider that variable.
Hope this helps (and wasn't obvious)