Help me out here, I am assuming by MWB that you mean Manual White Balance. How are you doing that manually underwater? To do this accurately you need an 18% Grey Card shot next to what you intend to photograph. This would mean you would have to change your settings and redo this every few feet before shooting as your color temp will be changing constantly. You are better off leaving it set to AWB and shooting RAW (NEF). THis will give you the file unprocessed by the camera settings. And one other thing this will allow you to do, you can set your shutter speed to a faster setting and be able to save it in camera raw. I have been able to save images that were 5 stops underexposed due to using RAW. If you have CS3, you can import your JPEG files into raw and save them, to a small degree. If I can be of any assistance with Photoshop, let me know, I use the program 6-10 hours per day. Good Luck!