Try and get hold of a
WhiBal 18% grey slate. (Or chop up a PADI DUP slate ;-)) They work slightly better than white slates. I just got the small pocket sized one and drilled 4 holes in the corners so i can bungee it to my left wrist.
Set the shortcut button to Set Custom WB if you haven't already... This allows one click setting, without even looking at the camera
Just point it at your wrist slate, click shortcut button, done... take photos. Do that after changing depth.
I use custom wb in P, Av, Tv... but i use the Underwater scene for shooting upwards. great for silhouette shots of your mates above you... Custom WB doesn't work when shooting upwards as there's a lot more red light reaching the camera directly than there is bouncing off the reef, etc around you, so you'll get pinkish shot even when you've just set the custom wb... And the UW mode in the S95 does give you nice blue water sometimes when shooting up, which is often what you do want ;-) (set the exposure compensation to minus a little or a lot for better results)
Custom WB also doesn't seem to work too well with fisheye wide-angle dome lenses, as they often have the surface in the shot, and therefore lots of direct light making that area pinkish again :-(
I often shoot several shots of the same thing with both custom wb and the uw scene, especially when shallow, and you often don't know which one will look the best until you look on your laptop after the dive.
I have an S100 now, and i think Canon have been playing with the UW wb, as it's often better then custom WB, when i'd expect it not to be (like most of the time!) And it doesn't seem to be as good for the upwards shots... But i need to do some A/B test shots to say for sure.
