From http://www.wsu.edu:8080/~brians/errors/sea.html:

In Shakespeare's Tempest, Ariel deceitfully sings to Ferdinand:

Full fathom five thy father lies;

Of his bones are coral made;

Those are pearls that were his eyes:

Nothing of him that doth fade

But doth suffer a sea-change

Into something rich and strange.

This rich language has so captivated the ears of generations of writers that they feel compelled to describe as "sea changes" not only alterations that are "rich and strange," but, less appropriately, those that are simply large or sudden. Always popular, this cliché has recently become so pervasive as to make "sea" an almost inextricable companion to "change" whatever its meaning. In its original context, it meant nothing more complex than "a change caused by the sea." Since the phrase is almost always improperly used and is greatly over-used, it has suffered a swamp change into something dull and tiresome. Avoid the phrase; otherwise you will irritate those who know it and puzzle those who do not.

-- LesOrchard - 10 Apr 2002