This is true at ISO 400 with the current generation of cameras. It is not true for the 1Ds generation of cameras nor when comparing the 1Ds to current DX cameras. If we assume like generations and like resolutions, then I think it's safe to say this although it may not always be ISO 400! Had Canon used 14 bit converters in the current generation, we'd be talking about this differently.
If you underexpose a shot with a DX sensor camera at ISO 400 you have at least one less stop to work with to bring the shot back if you aren't tolerant of noise. This one is easy to show examples although I don't have any though since I don't shoot DX.
High ISO hasn't generally been valued underwater in the past, nor has it really even been an option, so I don't accept that high ISO has suddenly become important. Instead, I believe its a generational luxury enjoyed by one brand. I recognize that it may be an advantage for wide angle FF shooting underwater because of the desired to maintain higher f-stops. Keep in mind, though, that it's the characteristics of FF that motivate those higher f-stops in the first place. It's smart for FF shooters to take advantage of the capabilities of their systems in ways that are not immediately obvious.
For commercial photography, shooting at ISO1600 is often critical. I have shot shows at ISO1600 and f2.8 and still had underexposed shots from the 1DmkII that could be brought up and were perfectly acceptable. If I'd been using DX I'd have had to run a noise suppression program - adding an additional step to my workflow.
Anyhow, this is sort of a general post based on my experience. As most of us user our underwater cameras as general workhorses (shooting topside and below) this is still important stuff.
Craig's comments are pretty narrowly focused where there is literally no difference in performance. Shooting underwater macro at ISO100 you can get the same results with just about any system.
I don't agree that it's right to dismiss underwater values because of the assumption that we're all committed to topside photography. This is an underwater forum so it's reasonable to emphasis underwater uses. What's important topside should not dictate what we use underwater although it's clear that many will value both. I do know someone who shoots Nikon below and Canon above!
Finally, my comments are not focused narrowly, they are focused on specific capabilities of specific cameras and how those capabilities contradict claims made here. I'm not offering any reason for one to choose one system over another but refuting one that's not well thought out IMO. I do believe, though, that my arguments ultimately suit the vast majority of situations underwater. Arguing that DX can achieve tonal parity with FF is hardly an endorsement of DX! It is, instead, an argument that the real differences come down to the lenses.
I agree. When it was initially pointed out to me that 1D3 has fixed my biggest grievances with the the Mk2's, I gave serious consideration to that body. Unfortunately, the housing manufacturer I wanted had no plans to support it. That should change with the 1Ds3 but I can only buy what's available now! I believe the 1D3 will make a great underwater camera, perhaps better than the D2x. Of course, if we're speculating on the future, I believe both the 1Ds3 and D3x will be even better. I believe 14 bit will make a difference.
I like the D2x underwater quite a bit, but for topside, from what I have read to date, I think I'd prefer the MkIII. I think resolution disparities between 10 and 16 mp, is a red herring given the other shortcomings digital capture needs to corral first.
As usual, Alex, you've said what I think is really important to say regarding FF and DX for underwater. I have my 10-17 now but no 1.4x converter that will work with it. I think it'd be fun to try that combination just to see what its characteristics are. Put that combination on full frame! I have a bit of Rube Goldberg in me...
I think many wonder when Canon will follow up the 30D. That would be reasonably be a DX equivalent to the 5D for them and a competitor to the D200. I wonder how many underwater shooters would consider it over the 5D?