The misuse of it has been going on for a long time. Fowler complains of it as a hackneyed phrase, though I can't remember whether it's in The King's English (1904) or Modern English Usage (c. 1926).

As far as I'm concerned it's a misquotation pure and simple: those two words do occur together in Shakespeare but not at all in the sense now intended. Ariel's sea change was a slow transformation by the corrosion of time - not an abrupt reversal as if in... mid-stream?

What do people think they're saying? Even if they think it just means "change", why say "sea change" instead of "change"? "There has been a (sea) change in this government's policies"... what does it add?