I've made a YouTube tutorial about how to take snap-shot quality stills pictures with a GoPro Hero4 (with no lights).
David Newman of GoPro was kind enough of sanity check the method. I recommend 2.7K resolution, but he suggests going all the up to 4K because no light would be lost. On a Hero4 Silver that means a 1/15th sec shutter, too long for me. But if you have a Hero4 Black or a steadier hand, 4K should work.
I'd appreciate suggestions of Internet scuba forums that I might want to post to.
Thanks,
Carl