In 1911, the U.S. Coast and Geodetic Survey started publishing information on the Intracoastal Waterway Route because “many waterways formerly but little frequented are now extensively used, and the great development of motor boating, has caused an extensive demand for accurate charts of the inland waterways…” (Report of the Superintendent of the Coast and Geodetic Survey showing the progress of the work from July 1, 1910, to June 30, 1911, Superintendent O.H. Tittman)
The first known appearance of what is commonly referred to as the “magenta line” is in a set of eight charts (each titled “U.S. Coast and Geodetic Survey INSIDE ROUTE”) included in a now-defunct U.S. Coast & Geodetic Survey publication, Inside Route Pilot, 1st edition 1912. An example of the Inside Route charts is this 1913 chart showing the magenta line from New York to Key West.
Once surveyed and charted, the Intracoastal Waterway Route remained much the same until 1935 when, due an influx of resources from the Public Works Administration, USC&GS was able to re-survey and re-chart the route. An example of the Intracoastal Waterway chart is Chart 834, from 1936.
For more information, see Background ‒ History of the Intracoastal Waterway Route “Red Line” (PDF).