Peter's methodical approach is probably the way to go. I had about a year with the A7s and I am convinced the camera could not MWB consistently. I get inconsistent ambient back ground colour. I would suggest stay away from PP7 and S-log for UW. Even with a Shogun recorder. However this combo works best topside. I almost ruin a day's shoot at Northern part of Palau with this setting. Very washed out but where as on land this would be okay in post but there is something UW I did not anticipated. You need ND to stop-down as the camera ISO is fixed lowest at 3200. Not good on a bright morning and water full of mantas near the surface.
I find the A7s works best close up and with dive lights. Superb results with macro. Set it custom profile that you like and expose properly. Never forget those ND filters. My two cents.