Obviously, you'll get a variety of opinions from different folks. For what it's worth, I'll add mine to the mix.
I currently shoot underwater with a D7100. At some point, late in the fall, I'll send my Ikelite housing in to have the strobe hardware in the housing upgraded so I'll be able to use both my D7100 and D7200 camera bodies underwater.
If you can swing it financially, get two strobes. Two strobes are better than one; one strobe is better than none. I seldom shoot without the diffusers on mine. Get long strobe arms; these will give you more options for lighting than short ones.
My underwater rig has three lenses. I've got a Tokina 11X17 Fisheye, Nikkor 60mm Micro, and a Nikkor 10X24. The lens that sees the most use is the Nikkor 10X24. The optics of this lens are incredible.
Many really like the Tokina 11X17, but I'm not a big fan of the curvature one gets with the fisheye. Of my three lenses, the Tokina sees the least usage. The Tokina is a good lens, but I just like the results I get with the Nikkor 10X24 better. I'll be doing some work with the Tokina next week in Hawaii.
I enjoy chasing wide angle images better than doing the macro thing with my Nikkor 60 Micro. The 60 has produced some remarkable images.
Believe it or not, I've been in situations where I was rigged for wide angle stuff with my Nikkor 10X24 and found smaller critters that I just had to try to get photos of. I've been able to get usable images by changing the image area from DX to 1.3 in the Setup Menu. Check the sea horse images on my website: www.cortezbluephotography.com and see if you can figure out which ones were taken with my Micro and which ones were taken with the 10X24. Keep in mind, I think a macro lens is best for shooting macro and wide angle is best for wide angle; still, sometimes, one has to get what you can with what you've got.
What lens (wide angle or macro) you choose if you can only afford one lens to begin with really depends on what you want to shoot. I shoot with other photographers who started with a macro lens. If I had only had been able to purchase one lens when I started, I would've opted for the 10X24. Wide angle stuff has more appeal to me.
-Tinman