I just came back from a dive trip to Cozumel and tried my new D800's video capabilities.
I'm happy to report that, with color correction in FCP, the D800 does a great job without a red filter or lights.
I didn't have much chance to experiment with this--too many other new things to try--but here's a quick preview.
This is an uncorrected frame shooting without lights, auto white balance and programed exposure and shutter speed in about 40 feet of water (sunny day topside):
Before correction.png 396.86KB 179 downloads
I did a quick and dirty color correction using the three way color correction tool in FCP. Not surprisingly, the corrections added red:
I also tested use of the Magic Filter for a few dives. I did not get better resuls, and worse, I had to do a lot of color correction on my stills to make them usable.