I recently took the Nauticam BMCC prototype to film Great Whites at Guadalupe and I've just posted a short comparison video between the BMCC and PMW200. (This You Tube version is pretty blocky and the banding is pretty bad, but I'm re-encoding and I'll upload another version when I can and also upload to Vimeo).
I'm still writing down my thoughts on the BMCC. I only had the equipment in the water for a limited time in not what you would call 'normal' dive conditions, but I guess in summary I really liked the Nauticam Housing, but had some reservations about the camera for underwater shooting.
I think if you have the ability to shoot 100% in RAW and the time/budget/proficiency for RAW post production then some of my fears would be overcome, but that shouldn't be dismissed lightly in terms of the amount of work that is required.
I'll post the full write up just as soon as I've finished.