Some areas of the US East Coast's Intracoastal Waterway regualarly fill up with sand and silt from the small tributary creeks and rivers along its length. Certain sections are actually shallow rivers, themselves. This constant water action builds up sand bars, expands them, and sometimes even causes them to migrate from one point to another. In some areas the shoals are so active, nobody bothers to mark their precise locations on charts or in guide books. If the Coast Guard or a private entity marks the general location of the shoals with a piling, float or buoy, it's a start -- but it may still take some work to identify certain shoals that are well-known to the locals but not to folks just passin' through.
Case in point: the large crescent-shaped shoal in the Matanzas River at St. Augustine, just south of the Bridge of Lions and less than one boat length east of the municipal marina's south mooring field. This shoal extends southward about 2/3 of the length of the entire mooring field and wraps around its southern end. The shoal is 3 feet underwater and cannot be seen in almost any condition - until your boat hits it, that is. No charts show this shoal, not even the St. Augustine municipal marina's printed chartlet of their mooring field or the marina's web page. The marina will, however, advise you how to avoid it if you ask them about it. As if you would ask about an uncharted shoal you do not know exists.
Active Captain's satellite image of this area shows the shoal, and Active Captain links you to this .pdf from the St. Augustine's government website showing the shoal (in case the link is busted, it's http://www.staugustinegovernment.com/visitors/documents/FinalmooringplanSandM.pdf).
Boats of all sizes run aground here several times a week. That pic of a sailboat up there, behind that moored power boat? They were moving outside the mooring field when they got underway. That's usually the safe way to operate a boat. They grounded hard on that invisible, uncharted shoal, and they stayed grounded for several hours until the next high tide. Look at them list to the side as the tide goes out. Potentially that is very damaging to a boat - let alone what the grounding might have done to the keel. In this case the boat eventually got back afloat with apparently no great harm done. Which: Good for them!
What should they have done to avoid running aground? The very counterintuitive thing: (1) know that the shoal exists before you hit it; (2) travel northbound in the deeper water of the mooring field itself, amongst dozens of boats, until you almost get to the Bridge of Lions, then (3) make a 180-degree turn around The Unknown Shoal to head south.
Technically, this shoal is outside of the ICW proper, but shoaling like this occurs at many points along the 3000-mile length of the ICW. I do not like the ICW. Or the shoals next to it.