Thanks for all of the replies. I understand the concerns about
liability. The issue is that there are several large rocks in the
river that no one can see. Many people go zoming along try to avoid
them, but few succeed. Yes, you try to learn where they are, but
they're so scattered in the area around us that it's like a minefield.
No matter how fast you're going it brings the boat to a grinding halt
and nails the prop - bad news.

I was merely trying to save a few engines, the bottoms of some boats,
and the occasional whiplash. Like I said, it's not "my river" to mark,
but I was just trying to be helpful. At the same time, I understand
the liability of marking a "channel" and being wrong and having someone
try to sue me, even though they could still very well pick their own
route and run right over the buoy.