How deep was the water indicated by the nautical phrase “mark twain”?

The nautical phrase “mark twain” indicates that the water is two fathoms deep, which is equivalent to 12 feet (3.7 meters).123

In this term, “mark” refers to a measurement, while “twain” is an archaic word for two. This phrase was used by riverboat pilots on the Mississippi River to signal safe water depth, and it famously inspired the pen name of the American author Mark Twain, who worked as a pilot in the late 1850s and early 1860s.132