The D200 is a nice setup - that's my primary setup. I have also a D70, which is also a great setup. D200 allows me a bit more latitude in cropping with the slightly higher resolution.
Remember also that megapixel count is definitely not the be-all or end-all of looking for a good camera. I think sometimes people focus a little too much on megapixels - but it is a major marketing point, though. Remember to double your resolution you need 4 times the number of megapixels, so the difference between an 8 mpix and 10 mpix camera from a resolution standpoint is negligible. I'd rather have better dynamic range than a few more megapixels.
There are a number of great housings available. Ikelite makes a great housing for the D200 - I have an Ike housing for my D70. I went with the Aquatica housing for the D200 - one of the reasons was that it was the only one I found that was rated to 300 ft. Plus it was nicely designed too .
For lenses, I would say the 60mm micro would be a good first macro lens, like Mike mentions. The 12-24mm is a decent wide zoom, affording some flexibility in the focal length. With a mid-range zoom I tend to think you do tend to sacrifice something to "have it all", ironically. Not that it's necessarily bad, but my personal preference is to go with a specific lens for a specific job.