Generally Rokinon lenses are all Manual Focus, so not really recommended for underwater as you don’t want to deal with focus issues with everything going on. You would have to use a very small aperture, and tests several focus points to find the best one, every time pulling the camera out of the housing and back in. Frankly, it’ll be a drag.
Something like the Tokina 10-17mm would be much better to use.
The Tokina 10-17mm is easily supported with the Nimar housings under this dome port:
Ran is a professional photographer for over 14 years. His passion for scuba diving and photography has pushed him to combine his profession and hobby and become a professional underwater photographer. Teaching is one of his greatest passions and over the years he has shared his experience with many divers and aspiring photographers. Along with his wife Danielle, an experienced Scuba Instructor, they have founded Dive and More, leading dive trips and UW photo workshops all over the world. Ran is also an electrical engineer and an avid internet marketing specialist. Visit his personal portfolio at www.ranmorphoto.com.