While running up and down the Intracoastal Waterway (ICW) this past year, I watched accuracy of our charts and GPS very carefully - I use the computer as a chartplotter; the 17" screen provides very good resolution and I can see just how far "off" we are relative to chart and GPS. The charts I used were downloaded from NOAA just a month or two earlier and had the most current corrections. Even so, many, many times channels defined by fixed daymarks (as opposed to floating buoys) were not where the charts said they were. Though perhaps only 100 to 300 feet off, if one were navigating strictly by chartplotter rather than using one's eyes to verify the position of the marks, one could run aground or smack dab into one of those daymarks!
Here's a reasonable short guide to Using Nautical Charts with GPS