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Seriously ?
The stock lens is approximatly 170 degrees on the diagonal in air but that reduces to around 128 underwater. Not sure if the OP is wanting to mount an inon ufl-165ad or what. Would probably require shooting on narrow mode to prevent vignetting so I'm not sure there would be much of a benefit.
You wish. The hero2 that was the same 170 degrees on land in water is narrower than my Inon UWL with 100 diagonal, in fact the gopro hero2 is around 90 diagonal or 80 horizontal.
I have seen hero3 footage compared to the same lens and it is clearly narrower and it does not get to 90 degrees horizontal
It has a wide angle lens in a flat port. It HAS to be narrower in water. 90 degrees FOV sounds about right.