Hi! I just started using the Rokinon/Samyang 7.5mm fisheye with the Nauticam 4.33 dome port. This is in combination with the Nauticam housing for the Olympus EM-10. There is almost no vignetting with this set-up.
However, corner sharpness can be a problem. Since you can't change the aperture during the dive, I usually just leave it at f11 for better corner sharpness. Still not that sharp, but (in my opinion) acceptable for a fisheye lens. I just adjust exposure by playing with the SS and ISO.
What's more difficult is maintaining proper focus during the dive. I was able to mount the Nauticam zoom gear for the Olympus 9-18mm on the Rokinon using thick rubber bands. I use this, in combination with "focus peaking", to adjust focus during the dive.
I'm still experimenting to get the best settings. Hope this helps...