I'm reading an extensive biography of Matthew Fontaine Maury, by Francis Leigh Williams, Rutgers University Press, 1963.
In a chapter regarding mapping the sea floor, MFM, in a letter of September 4, 1858 to the Secretary of the Navy
, comments, "These were the first maps of the kind ever attempted for 'blue water'".
Author's notes: The term "blue water" was already in use at the time which explains why MFM was comfortable using the term. Blue water
was a nautical term for the deep water
of the ocean opposed to the shallower waters close to shore.
This seems to support my understanding that the term applies to away from the continental shelf. With respect to sailing, the term applies to 'other than coastal' passage
making. I thought this historical reference fascinating especially in light of the current
day contention of the term.
Side note of interest: MFM was hired by Samuel Morse to map a route
for the first undersea telegraphic cable. The cable was to run from Newfoundland
to Liverpool, England
. MFM was speaking of the abyssal plain and the curious discovery of a mid-oceanic rise.