Clams are not always happy

The full phrase is "happy as a clam at high tide."

The phrase originated in the north-eastern states of the USA in the early 19th century. The earliest citation that I can find is from a frontier memoir The Harpe's Head - A Legend of Kentucky, 1833...  The expression was well-enough known in the USA by the late 1840s for it to have been included in John Russell Bartlett's Dictionary Of Americanisms - A Glossary of Words And Phrases Usually Regarded As Peculiar To The United States, 1848:

"As happy as a clam at high water," is a very common expression in those parts of the coast of New England where clams are found.

Text from The Phrase Finder.