NRSS,
I highly suggest you look at the Fuji S2pro body as an alternative to the D100. It uses the same body/lens mount, but has a few other features that make it a "better choice" for underwater use.
I am biased of course, because I use the S2 and love it. If you care to read a review about my experience w/ the camera and the Ikelite housing click on the link for "Reviews"
You will be able to use most of the lenses from you N90S system but you may want to try a different lens on the wide side. Believe it or not the 16mm fisheye or the Sigma 15mm fisheye are very popular.
For strobes - you use any strobes with the S2 in TTL, but you are limited to manual strobe control with the D100. In that case, you MAY want a strobe with more than 3 power settings.
hope that helps,
James Wiseman