The story
Clams live buried in the sand and mud of tidal flats. People dig them when the tide is out and the flats lie bare. When the water rolls back in, the clams are covered, out of reach and left in peace.
That is the usual explanation for the longer saying, “happy as a clam at high water.” It was in print by 1836, and in 1841 a newspaper in Bangor, Maine, wrote of “an interesting, and, undoubtedly correct explanation of the expression: ‘As happy as a clam at high water.’”
Yet the short form is the older one in print. An 1833 story set in Kentucky says of a contented man, “He was as happy as a clam.” Whether the tide was always part of the joke, or came along later to explain it, no one knows for sure.
Either way, it is an American saying, at home along the clam flats of New England. Today the high water has mostly drained out of the phrase, but the contentment stays.
