Visibility can be measured formally with what is known as a Secchi disk, which is a disc divided into black and white sections. Viz is defined as how far away you can distinguish the two areas from one another, but almost nobody goes to those lengths to determine was viz is. Other than that, the determination of visibility is somewhat subjective. I personally don't consider that I can "see" someone unless I can recognize who they are -- just because I can glimpse tanks or bubbles doesn't mean I can really "see" that far. I have to be able to make out details. And then you have the issue of trying to estimate how far away the person whose details you can see really IS . . .
I suspect that what I hear around Puget Sound is kind of along the lines of: if it's REALLY good, it's 40/50/70 feet; if it's pretty good diving, it's 25, if it's typical diving, it's 10-15, and if it's so horrible you wonder why you're there at all, it's "THREE FEET!" or less.