Back in 'film days' I tried to do underwater photography with a couple of Nikonos cameras (a II and a III). I didn't get to dive enough to really get any decent 35mm slides. While I still have the Nikonos cameras, I opted for a Nikon D7100 when I relocated close enough to the Sea of Cortez and Southern California to get into a serious, expensive underwater photography addiction.
I've no experience with the D5100 or D7000, but the D7000 was a top pick by underwater photographers who were using DX Nikon cameras before the D7100 came out. That's worth considering if you're making a decision about which of your bodies to put in a housing.
Keep in mind that with a Nikon DX camera body you're not really shooting the focal length lens you stick a non-DX Nikkor on the body. In other words, a 105mm Micro Nikkor N really isn't a 105. It's bigger. The crop factor on your D7000 sensor is probably in the neighborhood of 1.5. I use a 60mm Micro Nikkor N on my D7100 and it functions as a 90mm.
In addition to the 60mm Micro Nikkor N, I shoot a 10-17mm Fisheye DX Tokina and 10-24mm DX Nikkor. Believe it or not you can get up close and personal with a fisheye and a wide angle zoom. Take a look at the photos on my website and see if you can tell which lens I used to get the macro photos. I'll give you a clue. What do you do when you are rigged with a 10-17mm fisheye for big animal encounters and discover the dive boat has anchored right on top of a bunch of seahorses at a site where no one has seen seahorses before? If you're interested or just curious, take a look at the photos at
www.cortezbluephotography.com . Seahorse photos are currently in the Recent San Carlos Trips folder on the Portfolio page.
I shoot two strobes, but could probably get by with one for macro stuff. Still, two strobes on long arms do work very well.
I'm by no means an expert and I learn something new every time I go underwater to shoot. If I was really loaded, I'd look for a housing for my Nikon D300 and rig it for macro stuff and have the D7100 rigged for wide angle stuff.
-AZTinman