I just got back from a trip to Bonaire with the GH5M2. The battery life is extremely more improved. With the GH5 I could get 1.5 dives out of a Panasonic battery. The new GH5M2 battery gave me 2 100 minutes dives no problem. This is great for not having to pop open the housing as often. And, in Bonaire you don't want to keep extra batteries or anything in the truck for fear of theft. So, this is a great improvement.
I did three dives on one battery. But on the third dive I did shut down the camera when not using it. I have the camera connected to a small HD monitor.
Focusing is much faster and more accurate. Using back button focus was very quick and I seldom had to make multiple attempts.
I shot all video so nothing to report on stills. Once I review the footage and put a video together, I'll be more informed about those aspects.
Attached screengrab from the video.