Both my D200 and D800 are housed in Subal. It's taken a while to get the D800 housing dialed in so that all the controls work correctly, etc. but now that it works I'm pretty happy with it. I only have a handful of dives with the D800 in a working housing so I haven't arrived at any conclusive opinions yet. I have shot the D800 some on land and like it there. As far as I can tell I don't have the left focus issue though I haven't tried any kind of bench testing to prove it conclusively. With good glass the images are very sharp, even 1:1.
Noise looks great to my eye. I suppose it's a bit grainier than I'd expect at low ISOs if you pixel peep but there are so many pixels that I doubt that is a real issue for a final product. And I can shoot the D800 at ISO 1600 and get natural light shots I would never have dreamed of with the D200.
Underwater has been a mixed bag so far. Mind you I shot the D200 for 6 years and the D800 is new so I'm still getting used to it. I also don't have much experience with any of the other pro models like the D3S or the D4 so areas where I'm impressed with the D800 may not be as impressive to somebody who is used to an uber camera.
With the Subal housing and the DP-FE4 dome port I've found that neither my 16 Nikkor FE nor my Sigma 15 FE DG are particularly sharp in the corners. It isn't awful but noticeably worse than my Tokina 10-17 was at 10 on my D200. The Nikkor is worse than the Sigma. The 105 AFS-VR is sharp across the frame so I'm happy with that lens. I haven't tried any rectilinear wide lenses yet.
So far I'm finding myself a bit frustrated with the AF on the D800 (interestingly it has seemed mostly fine to me on land). It's possible this is just because I'm so used to shooting the D200 but I'm still struggling just a bit. The focus point grouping modes don't seem to work quite the way I want. For wide I want to be able to pick an area of the frame and basically have the AF pick the point that is closest to the lens to focus on. On the D800 it still seems to give strong priority to the one single point that is selected so I've missed focus by a small amount on a higher percentage of shots than I'd like. Meanwhile on macro the D800 certainly needs a lot less light to AF than the D200 which is great. Unfortunately I find myself having trouble getting it to focus on small parts of the frame like a nudibranch rhinophore. Even if the focus point is on the subject it tends to want to find things in the background to focus on. Hopefully I'll get used to it and it's more user training than anything in particular with the AF design.
Finally - for macro I find manual focussing to be much easier with the D800 due to the much improved (full frame) viewfinder.
Nikon D800, Subal Housing, Sigma 15 FE, Nikkor 17-35, 105 VR Micro, 2x YS-350, ULCS arms, LMI Sola 800 Photo, Macromate