I've called several dives ... the most recent one about three hours ago, due to lack of visibility. We were supposed to be doing a class dive, but when vis only allows me to see about half a body length, it's hard for me to evaluate what's going on. So we thumbed it. My student received it rather well, actually ... we're going to try it again in a couple of days. Hopefully by then this plankton bloom will have self-corrected a bit.
I've done the same thing - hard to evaluate a students' performance when you can only be sure where they are by hanging onto their shoulder straps!
I've called a fair few dives over the years. I've also NOT called a few dives, and I've got to say none of them got any better for the decision to tough it out...
'Any diver can call any dive at any time with no questions asked and no blame attached.' I'd guess most people don't run into that statement until they start technical diving, but it's something I tell Open Water students. If you're not happy being there, don't be.