cloudswinger
Contributor
It isn't like skiing where the steepness of terrain combined with condition management (grooming) equals greencircles, blue∏squares, or black◊diamonds.
To some extent, the characterizing of dive sites in this manner helps sell dive guides and maps. (After all, there are no pine trees to post trail signs on at the top)
The ratings in Skiing are relative to that location. A black diamond in Colorado can be significantly different than a black diamond in NC. And the main difference between blue and black is not the steepness, but whether it's groomed. I don't know who rates dive sites, but there probably is a bit of that same relativity to it also.