I have limited experience with the A7, Nauticam, Nikonas 15mm, and Zeis 24-70mm. Really like the camera and once I got dialed into using the 15mm really liked it. The 24-70mm was good for shark shots and I found the auto focus to work fine. Now that the 16-35mm and 90mm macro are coming out this becomes a better UW system, IMHO.
Most of the wide angle shots in this Cocos Island gallery were taken with the a7 & Nikonas pairing. Leave for the Solomons in two weeks and will give it another go.