I currently use a Nauticam 45 viewfinder on my Nauticam housing and LOVE it! It does take some getting used to though….but once you "get it", you won't' do without it. I'm actually switching from a Canon rig to a mirror less rig and am BUMMED that my Nauti 45 viewfinder will not work on the Nauticam EM-5 mark II housing. Therefore, I am selling my VF if you're interested - i have a posting on Wetpixel for it….
I wouldn't be so quick to sell it. The Nauticam writeup for the Mk2 housing mentions using the enhanced viewfinders and optional replacement window.
"Nauticam has crafted an optional rear acrylic display window that includes a mount for installation of Nauticam’s acclaimed 45º and 180º magnified view finders."