You really have nothing to worry about the file quality of the Sony RX100iv. If you download photos from our Smugmug site you can look at the Exif data and find the ISO setting of the photos. For example the Whale Shark photo (1st one in this thread) was shot at ISO640, and some other wide angle photos were shot at ISO320. My personal feeling is the files from the RX100iv is even cleaner than the EM5 Mk I.
I can usually use 1 battery for 2 dives.