Please report the setting you are using so I can reproduce on my set up.
I always set the lcd to sunny (max brightness) when underwater.
In manual mode and forced flash the lcd does not dim at all changing time, stays bright from f1.8 to about f7.1, dims from f7.1 to f9 and this is more evident only at f10-11 but it is never dark
(f9 on 1 inch sensor being f16 on full frame)
Using manual focus the effect of changing aperture is more evident and the image become pixellated , probably Sony wanted you to be able to evaluate the deep of field.
If you want manual focus , deep or dark water , macro f stops buy a very strong fosus light, I bought the FIT Led 2400 and I am using the nauticam lcd magnifier.
I set the lcd to sunny (max brightness)