What is the origin of the idiom "be in smooth water" and when to use it?

The idiom "be in smooth water" is rooted in nautical terminology. In sailing, "smooth water" refers to calm, undisturbed, and tranquil waters, which are typically easier to navigate than turbulent or rough waters. The phrase has been used figuratively in everyday language to describe situations where things are going well and there are no significant challenges or obstacles.