I recently sold my A7S and housing but still own the adapter that allows you to use the legendary Nikonos 15mm lens with the full frame A7 series. It is a wet lens and has the best IQ / corner sharpness I have ever seen for an uw rectilinear set up. Very compact for traveling as well (see: http://www.nauticamu...eea-of-cortez).The lens is completely manual and you control aperture / focus with two external knobs on the lens (focus peaking functions come in handy for this).
Adapter: http://www.backscatt.../na-37201.lasso($420 new)
Lens: 15mm Nikonos (New Style) - not sure what the market rate is currently (check ebay) but I picked mine up for $400 used on ebay.
I am selling both items for $500. You only need the NA-A7 housing and camera to use this combination. Both are in great condition, have only been used once.
Let me know / PM if you are interested or have questions.