I was a little bit surprised to find an electronic chart
of Alaskan Islands with the following footnote.

A study by the Canadian authorities within this area
indicates hazardous pinnacle-like features which may
reduce the depths to as little as 30 percent of the
charted depth. For example, where a depth of 16
fathoms is charted, a depth of 4.75 fathoms may exist
nearby. An echo sounder may provide little if any
warning when approaching these features.

Here is an intersting link I found related to this subject.

http://fatherandsoncharters.com/