Meaning and Origin of the phrase "Sea Change"?

Answer 1

Origin? Coined by Shakespeare in The Tempest: "Nothing of him that doth fade, / But doth suffer a Sea-change / Into something rich, & strange."

Meaning? "a radical change"

Answer 2

A sea change denotes a change that is of major consequence or transformation. Sometimes things happen or are made which have the effect of bringing about major changes in our lives. This is called sea change. For example, we say that 911 has brought about a sea change in the security situation in the country.
The origin appears to be related to the vastness of the sea with its effects (sometimes unexplained, in the olden days) which affected lives of people.

Answer 3

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;
Ding-dong.
Hark! now I hear them, - ding-dong, bell.

Shakespeare, "The Tempest"

A sea-change just means changed by the sea. It has more recently come to mean changing one's lifestyle by moving to the coast.