"Jeff" wrote
Nope. Don't work. Not Nohow.

but the boat is also going with the current. The boat speed is "through the water," not "over the bottom."

It will work. Where you go wrong is saying the boat is going with the current.
It is if it's dead in the water. When it's sailing it's also going with the wind.
You never want to give the wind any credit. So the current goes two or three
knots. And the wind makes the boat go double that. You can use the boat motion
caused by the wind to measure the current. You just have to go back and forth.
You don't need instruments to measure the wind. You can use Beaufort. The
wind becomes your reference point. It's better if you go back and forth at least
every forty-five degrees to really tell what's going on with the current.

The wind that the boater would call "true wind" is actually the vector sum of the wind and current (adjusting signs as
appropriate).

And if there's no boat around? There's still apparent wind if you have
a bottle drifting with the current.

nope. You're connected to the current while you're in the boat.

Only if your just drifting along. Once your sailing the wind makes your boat
move independently of the current. The current still affects it but it affects it
different in different directions.

Your boat measures the wind speed.

You just contradicted yourself.

And how do you tell the boat speed over the bottom, as opposed to through the water?

The bottom's got nothing to do with this. He just wanted to know if you could know the
current with no outside landmarks and the bottom is an outside landmark.

Cheers,
Ellen