I just shot with the 5D2 in the waters off of Long Island in near blackness, nothing but a headlamp and shore lights. I shot at 6400 and 3200 iso as planned but came away with more noise in the image than when I did tests. The camera was in an Ikelite underwater housing so I don't know if heat built up in the processor, but then again the water was 36 degrees so....
Anyone have a reco for noise reduction software/techniques in post?
There are many tools as the suggested by Robbie. Also you can try to apply some (very little) Gaussian Blur in the Green or Blue channel.
But if you look for a really pro de-noise tool, have a look to Neat Video. I've tried all the software solution, and this is the one that really works.