Upgrading from GoPro, Seeking advice...

RX100 VI and VII won't fit into the Sony housing - MPK-URX100A only fits models I through V(a), with an additional caveat of the first two models ending up with their lens offset from the port center, creating problems with wet lenses. If you want to house an RX100 VI, your options are SeaFrogs Salted Line, Ikelite and Nauticam. Due to its long lens, all of these use interchangeable ports, with a short port allowing you to mount a wet wide lens but restricting lens extension, and a long port allowing full lens extension for macro but vignetting at wide angle. SeaFrogs also offers a universal flat port that works for the entire focal length range but has no provisions for wet lens mounting, and several dome port options. Fantasea has a housing for RX100 VI/VII, but it has a fixed port that is limited to 24-66mm focal range.

However, if you're focused primarily on video, keep in mind that Sony white balance underwater isn't the greatest. It's not particularly relevant when you're shooting stills with strobes, but it is important for video, where you need to white balance inside the camera. For a primarily video shooter, a Panasonic LX10 is probably a better choice to take underwater than a Sony RX100. Its main weakness is wet wide lens compatibility - most wet lenses cause vignetting, restricting you to the heavy and expensive Nauticam WWL-1 - but that aside, it is, by all accounts, an excellent camera. Since it's been on the market for a while, it shouldn't be difficult to pick up a used complete rig at a good discount - for example, this set sold for $1050 .