The depths on the GPS plotter, and the locations of the very shallow shoal areas were extremely accurate. When the chart-plotter revealed a 21-foot deep passage through the reef to the Atlantic's azure blue waters, the depth plotter's depth finder and bottom contour information was dead on. It also revealed a 4-foot deep patch of coral that was about 50-feet south of where I passed through the reef, and that's exactly where it was.
Most of the time is accurate but you have to know that sometimes isn't and if you have a over confidence on that accuracy you can get in real trouble. It had happened to me several times, not to be in trouble, but the accuracy being very poor, not to say misleading.
That has nothing to do with the GPS accuracy but with the charts that served as support for the information on the map in your plotter. The earth is not flat and the chart makers used different projection systems to make them flat. A good plotter has on the menu a choice of about 20 different projection systems used to make maps, to permit an adjustment between GPS position and the position marked on the map.
Many just don't know about this, other used cheap plotters that only have the most common reference system and even the ones that have god marine plotters will find that sometimes the system used for making the map is not available on the plotter. Those at least will know that in that region they will get inevitable errors, some with hundreds meters differences. And I am not talking just about exotic places, that happens for instance in Croatia that is one of the most common sailing destinations.
Believe me, many don't know about this. The guys on the Croatian charter company did not know nothing about it or that the plotter they had in the boats had the wrong setting or even that their plotter had a menu with different settings for different map systems.