There is a technical definition for visibility that involves an object of a
specific size, shape and color that is, for the most part, useless to a
diver.  I do one of two things:

1. Even though visibility is supposed to be measured horizontally, sometimes
it's just too easy to do it vertically to pass up the chance.  Two weeks
ago, I was at 60 feet and could see the buoy I was towing (drift dive) on
the surface.  Visibility was 60 feet.  My dive buddy said that he wanted to
know horizontal visibility, so I told him it was 59.75 feet, a slight
adjustment to the original 60 feet just in case horizontal visibility was
different.  8^)

2. When there is a clear change in visibility as I descend, which is quite
common here, I'm more likely to assess visibility horizontally.  If I stop
to think about it, I think I'm pretty good at estimating distances
underwater.  If I don't stop to think about it, then my recollection of
distances is probably not as good, but still good enough to communicate a
general concept regarding visibility.

The point of all this is that, for most of us, on most dives, an
approximation that conveys the subjective idea of good, bad or indifferent
is close enough.  If I say it's 100 feet, it may or may not be, but you know
it's good.  If I say it's 30 feet, I'm more likely to be close to the actual
distance but what you really know is that visibility, for here, isn't all
that good.  If I say I could not see my guage with it held against my mask,
you know it's time to go somewhere or do something else.  If I say you can
see forever, you know I'm not diving in S. Florida.

Lee