I’ve always been interested in this quirky expression, which is used to describe someone who is very happy. The Phrase Finder explains “…that open clams give the appearance of smiling.” The full phrase is “…as happy as a clam at high water,” because high tide is when clams are apparently safest from predators. The expression is particularly popular in coastal areas of New England, but is used everywhere. The first citation found was in a frontier memoir dated 1833.