I bought a D3 a few months after it was introduced, and prior to the announcement of the D700. Full-frame capture was the primary attraction for me. But with the fast introduction of the D700, I wish I'd waited a bit longer. From my standpoint, as a film and TTL shooter, the D700 has one feature, lacking in the D3, which I feel is an over-riding factor. An independent Flash Compensation control. This would allow the user to vary i-TTL flash output WITHOUT affecting the ambient light metering. Both cameras have programable controls that would allow you to quickly turn strobe lighting off, IF the housing has a control for this button. This is worthwhile with regard to fast frame rates for a passing big fish when no flash is desired. If you have to reach up and turn off your strobe(s) manually this may take enough time that you miss the opportunity. A second issue is whether the housing allows allows a quick change from single frame shooting, which is more appropriate for flash images, to the highest frame rate for that ambient light sequence. If you just leave the camera set on the highest frame rate - 9 fps, you may fire off two or three or four shots each time you push the trigger for a flash shot.
I looked briefly at several D3 housings at DEMA. Briefly because I had already decided that the D700 is the camera I want to house. And don't recall that any of them had a control for frame rate.
I feel that looking at D200 and D300 housings gave me a good idea of the manufacturer's basic control design philosophy and a good preview of what a D700 housing might be like.
Fred