In Act II scene ii of Macbeth, what does Shakespeare mean when he says, "A little water clears us of this deed." What does it represent?

Water washes off Duncan's blood from Macbeth easliy. 
Macbeth, after bouts of inner struggle, has murdered Duncan but is now overcome with
guilt and fear.  Lady Macbeth must see to completing the conspiracy by planting the
dagger in the possession of unsuspecting servants.  Here the masculine and feminine
roles reverse in that Lady Macbeth is the aggressor and Macbeth is squeamish about the
act he just committed.  Upon her return she casually notices the blood on her hands, as
oppossed to Macbeth's obessession with blood on his hands, literally.  Lady Macbeth's
statement "A little water..." also contradicts Macbeth's earlier quote, "Will all great
Neptune's oceans wash the blood from my hand? No..." The quote in question clearly
reveals the Macbeths' approach to Duncan's murder.  Macbeth in his guilt is convinced he
will be found out, and Lady Macbeth in her ruthlessness is only concerned is hiding the
act.  Both equally guilty but obvioulsy two opposing attitudes toward the crime. Macbeth
cares that he is guilty. Lady Macbeth, though only an accomplice, is unfeeling that any
wrong doing has been done.