[Cussy]

David - Thanks for sharing those. I notice the second was taken at a 1/400th, so a slower shutter speed and higher f number might have helped (although you then might get blurring), but on the first image you don't have so much latitude. What does "lightly optimised" mean? Sharpening of images can often make the noise appear worse. What I tend to do is remove the noise first then sharpen. If the red spectrum was seriously lacking, which in natural light photography it always will be less than say 5m, then a readjustment of red will result in a lot of noise. Manual WB is just doing this for you, so the noise is most likely coming from the incomplete light spectrum. Natural light is definitely a time when capture in RAW is an advantage. You better hope those clever Russian's do a Fuji hack, just like those pesky Canon owners have

There are plenty of programmes that can deal with this sort of noise, Noise Ninja for instance. Your images just go to show we are at the absolute limits of what these remarkable little cameras can do. They aren't designed for underwater.
Rob