I have the Nauticam NA-6400. I chose this over the NA-6600 because of the built in flash to fall back on. The larger battery of the A6600 is partially offset by the Nauticam auxiliary battery. I have only had my new rig in the pool for testing, no actual shooting yet but very soon I hope. The IBIS is cool on the A6600 but I did without for years and many lens have OSS.
I went with the Macro Port 45 with zoom/focus and the CMC-2 semi-macro lens and the WWL-1 for my wide angle. I have the kit lens for use with both. I also have the Sigma 19mm f2.8 for matching with the WWL-1. I am also working with the old Inon UWL100 plus dome and the Rokinon 24mm f2.8 lens but it looks like this combo will need the Macro Port 35. I plan to also use the UWT TTL trigger but I am having unsolved issues there but at least the camera onboard flash works, lol.
I also have a Tokina Canon mount 10-17mm fisheye lens I bought several years back when I nearly went DSLR but decided it was too big and too much hassle to tote around. I like the Sony APS-C format and the camera is superb. What is missing though, and hopefully will be addressed someday, a little fisheye zoom lens in E mount. But we can use the Metabones and Tokina or even the Sony 16mm and fisheye converter. Some complain about soft edges with this combination but I have the little lens and converter and it seems decently snappy to me, thus far surface use only.
For the A6600, just keep in mind there is no built in flash so you will have to have a trigger system.
I went to several camera places and handled everything I could, none focused faster than the A6400/A6600. There are other considerations but a solid and fast AF was very important to me. I also do surface action and motorsports photography and do not want to have two different camera systems.
James