Amen on that, I have the same specific graphics issues, I have the same iMac setup; mid 2015, 27", 32 Gigs of Ram, etc. No hacks, no mods, Retina gfx (simply stunning color and depth in game) ... the general is I have opacity, screen issues in turn based... At sea, triggered during ship to ship boarding & combat, during which seems to trigger when mouse accesses any of the menus... also same gfx corruption issues in background areas of Delvers Row but at a far minor extent. At sea it's impossible to fight ship to ship as whole screen is either white or black with slight color line hints showing through - frustrating and impossible to play. Crashes on exit to desktop. Adjusted gfx down to lowest level, also turned off Night View - no dice. I know you guys will do your best to find and fix this - I cannot find my save file to send you. I will try to post a screen shot...
MD
Washington
USA