A sea change
A radical change or transformation.
From Shakespeare's The Tempest, 1610:
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.
Sea-nymphs hourly ring his knell
Shakespeare's usage incorporates the current meaning, that is, a radical change. He also made the expression richer by alluding to the literal meaning, that is, 'a change that is brought about by the sea'.
See other phrases and sayings from Shakespeare.