Throughout the play, the motif (recurring image, symbol, or idea) of blood is consistent; however, its meaning evolves. Blood at the beginning is associated with violence and warfare, Macbeth's "brandished steel...smoked with bloody execution" (I.2.17-18). But bloodchanges to guilt by the time the audience witnesses Lady Macbeth trying to cleanse her hands while sleepwalking in Act 5: "Here's the smell of the blood still" (V.1.53). The blood refers to the guilt of the crimes of which she is associated, the murder of the King in Act 2, the murders of the Macduff family in Act 4, even Banquo in Act 3. The blood isn't really there, but the guilt is all to real, hence how strong her imagination is and how strongly she feels compelled to phycicall wash her hands clean.
That brings us to water and water imagery, the imagery of redemption and resurrection. The water of Act 5 represents her desperation to wash herself of the guilt, to disassociate herself from the sins she has committed. In line 55-56, she even recalls the conversation she had with her husband just after he killed the King, entering their chamber bloody and scared, "Wash your hands; put on your nightgown; look not so pale..." The guilt has even pushed her to the point of talking to herself. She cannot shake this no matte how much water she may use to cleanse herself.