I purchased my Nikon 8-15mm last July and on my last dive on that year's dive trip shot it on a D800 in a Subal ND800 housing with a Seacam wideport on it.. Nice results, one of which, mating octopuses, I posted under this topic. I'm finally diving again with a new camera (Nikon D850), new housing (Nauticam NA-D850), and new port (a Saga 4" dome).
Enroute to Australia I stopped in Bali and was able to do five dives with the above combination in conditions ranging from mediocre to great (Kalanganyar, then Menjangan Island, and finally Secret Bay). I took several hundred pictures with the fisheye behind the Saga dome which does not allow you to use Nikon's own lens shade. I did have a shade on the Saga dome port that was made from a PVC pipe-joint that I purchased from Home Depot for $6.05 and the paid a machine shot to fit to the dome. The shade also provides protection for the otherwise exposed and vulnerable glass dome.
In none of my pictures did I get any of the unusual flare that Alex Mustard and Adam Hanlon reported. I enjoyed shooting pictures with the exaggerated perspective of the fisheye while getting quite close to subjects that were sometimes only a few centimeters in length.
The striped catfish at Kalanganyar were only about 2-3 cm long, but look larger due to the fisheye and being only about15 cm from the dome port. The Striped Cleaner Shrimp was also quite small and was only a cm or two from the Saga dome port. The yellow frogfish, Antennarius hispidus, was about 20 cm long. The first shot with the rope was shot from about a foot away and the second shot from only a few inches.
Although I was shooting FX with no cropping, it is easy to imagine the cropings that would have been possible had I chosen to shoot in the DX mode or 1.2 crop mode.