"Crossing: where the channel goes from one side of the river to the other."

I always thought the "crossing" meant the place where the steamboat
changed from one side of the river to the other, to be in slack water
going upstream, or to ride the main current going down stream. As I
recall, in the film "Life on the Mississippi," Sam is told to start his
crossing at a certain point.

Point (n.) Anything which tapers to a sharp, well defined termination.
Specifically: A small promontory or cape; a tract of land extending into
the water beyond the common shore line. [Websters Unabridged 1918]

Twain may have made the quote because he thought the term was river
pilot jargon.

--
Dick Ford