The scene depicted is from Shakespeare's Hamlet, Act IV, Scene VII, in which Ophelia, driven out of her mind when her father is murdered by her lover Hamlet, falls into a stream and drowns:
There, on the pendent boughs her coronet weeds
Clambering to hang, an envious sliver broke;
When down her weedy trophies and herself
Fell in the weeping brook. Her clothes spread wide,
And, mermaid-like, awhile they bore her up;
Sir John Everett Millais | Ophelia, 1851-1852 | Tate Britain, London