I usually estimate using body lengths. With fins, most divers are about six feet long. I don't count shadowy figures as visibility. If I can clearly make out what a shape is then I count that distance. On one amazing shore dive I actually measures lengths in the sand using my fin tips. We had 40 feet vis and it was the best I've ever seen on a shore dive in SoCal. When I read reports of 50-60' vis around here I just shake my head.