I've got two friends who are both saying that, when shooting underwater on their G10s, they must do a manual white-balance on a card because correction in Lightroom is impossible on auto white-balance, despite shooting raw. I've seen examples, along with earlier shots at auto which seem so bear out what they are reporting. We had all assumed that the white balance setting was only a metadata item in a raw file but this experience suggests otherwise. If I didn't know better, I'd say that the camera is performing some analogue gain adjustment during the custom white-balance. There's precious little documentation about the technicalities of the G10 in-camera raw image processing out there. Anybody have any thoughts or experience with this.
Taking a slate is a waste of time in my opinion. I find that a diverse coral head or sandy bottom or air tank is just as useful if one must set a white balance underwater.
Generally, I set the camera WB on "auto" underwater for decent previews and know I will correct in post. I do it by eye and with knowledge of the color of my strobes.
"Auto" WB in LR will only work in some very specific cases ... and not particularly well at all if there is a lot of water column in frame.
Are your friends shooting with strobes or ambient light? Any filters involved?
Usually easy enough to set the WB by eye in post if you shoot RAW and use strobes or filters.
Here's one shot with strobes and a second with filters and ambient light:
