Ferdinand, son of the King of Naples, is under the impression that his father died when their boat sank near an island. Sitting on the shore, he hears Ariel, the spirit whom Prospero commanded to sink the ship, singing nearby:
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.
Shakespeare meant "sea-change" a bit more literally, but as Bartleby notes, it now means any "marked transformation."
A nice little song, by the way, though I never really considered it an important part of The Tempest. A few hundred years later, T.S. Eliot stuck bits and pieces of it into The Waste Land, one of my favorite poems.