I was wondering about that but it would only work for the sailor standing in the right place on a boat sailing dead center of the river where there is typically two way traffic.

This doesn't seem like a utilitarian solution, more of a signal with a symbolic intention?

The law explictly requires "large enough to be conspicuous and by night a white light", which suggests it's about visibility, particularly at night. That makes sense, there is a black metal bridge near where I live, and when it is cloudly I've noticed it's suprisingly difficult to see, even when you know where it is.

Hm, I guess arches were lower back then, and a lot of the riverboats were actually the staked kind (think: Venetian gondolieri)? With different arches for different directions?

Honestly, this is all guesswork. But I can imagine something like that to be the case.