I still try to bracket exposures, as due to my vision problems, I often cannot view highlights or histograms adequately underwater.
I just rec'd this Nauticam LCD Magnifier from BlueWater Photo. I am really surprised at how much it magnifies the LCD screen! I can not wait to get it in the water in just under a week's time.
You simply attach the rail kit to the housing then the magnifier just pops on and off, and can be easily removed/replaced while diving. Your housing has to be Serial # A040732 and later for the rail kit.
The Fn button is very close to the hood of the magnifier, but it is not blocked. Just have to get the feel for where it is.