I got the D80 as I prefer the smaller lighter body for top side. It's always nice to pay a bit less for essentially the same quality images. The battery may last better on the D80 as its processing speed is slower but then someone else might know for sure - it has not been a problem at all so far.
However a few things did catch me out on the D80 in a Sea & Sea housing that were not at all apparent initially. The following cannot be altered underwater whereas on a D200 I expect they can.
1. Switching between AF-C and AF-S. For some reason you have to do this through a dedicated button on the camera top plate as there is no housing button. This feature is removed as a choice from the menu.
2. The same applies to Single and Continuous shooting.
3. The same applies to metering pattern - matrix, spot, center weighted
4. The same applies to front and rear curtain sync (may be the same as D200)
None of these are a headache for me but may be to others. It does mean I have to put the camera together very carefully with these settings pre chosen. a different make housing may have more buttons to get to these features.
After a D50 the image review features are superb on the D80 and I'm not sure if the D200 is as good. ISO setting and white balance is also very easy. I would probably get the same again and like SD cards.
It really pays to look in very close detail the finer aspects of contol.