So, in my quest for identifying my ideal underwater setup I'm currently thinking I should probably go with a Nauticam housing for my existing Nikon D850. Ignoring budgetary constraints, for wide angle (not fisheye) I could either get a wide angle lens - probably the Nikon 16-35/4, while muttering about it - and a dome port, OR I could locate an old 28-70 and go for the WACP.
I've read as much as I've found about the alternatives. It seems the WACP gives 2-3 extra stops of image edge sharpness compared to just wide-angle lens and dome, but on the other hand the Sea & Sea correction lens for the 16-35 gives about 2 stops also, and it seems possible to use that lens with a Nautical dome - at least the acrylic 8.5" and possibly also the 230 mm glass dome. If that's correct, the WACP has maybe one stop of edge sharpness, er, edge over the 16-35 with correction lens, which is no longer a huge difference (albeit possibly significant).