The book was written a few years ago but it's about events in Maine at the time of the American Revolution. My friend was reading it at his house on the coast of Maine (what we call "You Are There" reading) which sits on a cove with big tides. As he read, he would look up and see the retreating water and in the mud, two deep channels. We think these must have been the guzzles the book refers to but as you say, these are not all that commonly observed. We went straight to the dictionary -- print as well as electronic -- to no avail.