I'll let others debate the bodies, but I can't say enough good things about the Olympus 60mm macro lens. As long as you can get used to the effective 120mm length and you understand that it's really only good for macro, and not even for med-large fish portraits, it's just a fantastic lens. Sharp, portable, relatively affordable. If you're going with Nauticam and interested in even smaller critters, the CMC add-on lens is also pretty fantastic. Takes lots of practice, but the resolving power is incredible.
When I was in your shoes the biggest thing I wished for was examples so I could judge for myself. This gallery was all the 60mm macro:
It was on an EM-10 body, Nauticam housing, one YS-D1 strobe. A few of the shots use the CMC. I'll probably only ever get to Anilao once in my life and I was very happy with the little EM-10 + 60mm.