George,
I anchor up in the Merrimack river into the evening many times and
often (every other trip or so) will find someone cruising up that
simply does not see me, even though my anchor light is on. I ALWAYS
keep my spot light handy and point it straight up into the clouds,
before these people get too close. Once they see the spot, they turn
to avoid.
It's a (federal?) crime to spotlight the bridge of another vessel.
However, I would never leave the dock without a spotlight as finding
rocks and bouys, etc is too difficult without it.
-phil