My early thoughts on BMPC 4K from what I've read so far
+ Near super 35 sensor with approximate crop factor of 1.5 (wide angle shots are more possible),
+ Global Shutter which should produce better "motion" capture for the swimming creatures
+ Sharper image compare to the very filmy BMCC 2.5k which is a plus for underwater,
+ Better color science/rendition directly from camera and that means better productivity for color grading,
+ 4k is good to have now whether for compositions, downscaling for better quality full HD and for future proofing your captured great moments,
+ Wide selection of prime or zoom EF mount lenses,
+ Good value for money $4k for 4k resolution and
+ Availability of underwater housing form a good company namely Nauticam
- low light performance. But this relative to good performing cameras such as the Canon 5D mark III and BMCC. It must be better than my EX1. Remember this is a trade off for the global shutter. Fast lenses such as the latest Sigma 1.8 could help compensate for that
- less dynamic range, again this is relative as explained in the previous point. Do we need extra dynamic range than the acclaimed ~ 11 points for the 4k for underwater video. IMHO, I don't think so.
Again, those are some early thoughts that are battling in my mind may be to justify embarking on a journey with BMPC 4k. But let's see if more footage will justify such endeavor.
More thoughts from the community are welcomed