Ive had some recent experience with the RX100M2, and I'd like to share my experiences!
Great camera, this one. So tiny and great image quality. It is quick to focus and the images look pretty sharp!
Unfortunately, the biggest problem for me (and this one could completely kill it for use underwater in my book) is the exposure simulation or live view that is built in to the camera. This shows the user a simulation of the actual image that would be produced from the aperature, shutter, and ISO settings.
Underwater, the problem is that when you dont have any ambient light you dont see ANYTHING but a black screen on the LCD after you focus. This even happens if you use the fill flash mode. The camera will cut to a black LCD with a half press of the shutter. BIG PROBLEM! I have been searching for a way to turn it off, but it doesnt appear there is a way??? You are stuck with it!
The Mk2 has a neat little feature that would be great when taking macro shots- you can actually magnify the image to make sure the important bits are in focus. But when you activate this feature, it just zooms in on a black screen. Hardly what I would call useful in practice.
Even with a focus light- in the day time it is almost impossible to see what you are focusing on unless you are in a situation with LOTS of ambient light, and arent using a strobe to light the scene.
I havent had much problem with this on land, which is why it is so frustrating. If I had known about it before hand I would never have bought the thing.