When cruising near coastlines, knowing how to spot a shoal can prove to be an invaluable skill!
New shoals can appear along a coastline, especially after periods of heavy rainfall. The runoff will carry sediment out into the water and deposit them near the shore. These shoals won't appear on charts, but they do present a very real navigational hazard.
These shoals are easy to spot, even if the bottom is not visible. The trick is to watch the waters surface! As swells approach the shore, the enter shallow water along the beach and turn into breaking waves. Shoals along the coast will have the same effect on the swells, converting them to breaking waves and warning mariners of their presence.
This birds eye view of a river draining into the sea clearly shows how the waves break along the shallow water bottom contour. While sailing along at sea level, these characteristics are not as visible, simply knowing to watch out for an area where all the waves break can warn you that you are heading towards a shoal.