There is no official source (NOAA, USCG, USACE) which suggests that the magenta line should be treated as a chartplotter route.
The ICW was laid out in 1913. Chartplotters were not very accurate 100 years ago! To aid pilots running down the coast, a magenta colored line was drawn on the charts to indicate which rivers, creeks, sounds and canals were all interconnected to provide an inshore route between the Chesapeake and FL. The Magenta line was merely like Hansel and Gretel’s bread crumbs. It is merely a means to find the path. The captains in those days monitored their course with, skill, experience, and hand bearing compass to know where they were on the ICW. For most of its life the magenta line was not edited or changed.
With the development of GPS and chart plotters over the past 30 years some cruisers have assumed that the magenta a line is indeed a chartplotter route. It absolutely is NOT. It is meant as a trail of bread crumbs to indicate that as you head north in the St. Marys River you will bear to starboard in Cumberland Dividings, until it turns into the Cumberland River. The ICW bread crumbs then direct you to cross St Andrews Sound, to Jekyll Sound, and then into Jekyll Creek, etc.
While one can actually run along the magenta line most of the time, that is because it is running though bodies of water where it does not matter if you are 100 feet or 200 feet on either side of the line on the charts. Heck there are places you can be a mile or two to one side or the other and still be in deep water. There is no one sitting in an office monitoring the ICW hour by hour or day by day making changes to the magenta line. Channels shift and change. If you try to follow the magenta line as a chart plotter route you may find yourself on the wrong side of a navigation marker or even crossing over dry land. (Garmin lost a law suit in 2017 when someone was injured following the magenta line displayed on their Garmin chartplotter which lead them on the wrong side of a lateral aid.) NOAA removed the line in 2013 because it was being used incorrectly as a chart plotter route. The are now adding it back, but noticeably leaving it completely blank where the channel is in flux such as the troublesome inlet crossings.
In studying the ICW using NOAA charts, Navionics charts, and Garmin charts, I can tell you that there is no one single “Magenta Line”. The so-called magenta line is not always the same on all charting systems. Some charts show the line in places it was eliminated on other charts. Some charts place it differently. Comments in Active Captain and other social media sources should include the date of the chart and charting system being referenced. Comments regarding the ICW should reference some physical location such as a nearby ATON.
“The Office of Coast Survey announced …” (in 2014) “that future editions of nautical charts of the Intracoastal Waterway will be updated to include an improved “magenta line” that has historically aided navigation down the East Coast and around the Gulf Coast. Additionally, Coast Survey will change the magenta line’s function, from the perceived “recommended route” established more than a hundred years ago, to an advisory directional guide that helps prevent boaters from going astray in the maze of channels that comprise the route.”
The attached screen shot of the east end of Fields Cut shows R46A which will not appear on the most up to date NOAA chart 11512 as seen in the second screenshot of the Aqua Map display. The magenta line leaves this mark on the wrong hand. In both charts our track is close to the shore and does not follow the magenta line which runs directly across the shoal.