Medieval romances often associate women with water, so it's no surprise that Keats borrows from that tradition in "La Belle Dame Sans Merci." The problem with women and water, though, is that men who mess around with women end up getting all soggy and wet. According to this symbolic tradition, men are weakened by their contact with women.
- Line 3: Already in this line, the speaker is associating death and "wither[ing]" with a body of water: the "lake." And it's not just any body of water. Lakes, unlike springs or rivers, don't flow (or at least, don't flow quickly), so the water in them stagnates and can grow nasty algae and pond scum.
- Line 10: The unnamed speaker notices that the knight's face is "moist" with "fever dew." OK, so he's sweating because he has a fever. But where did he catch the fever? Look where that word "dew" is repeated…
- Line 26: The knight says that the fairy lady fed him "manna dew." "Manna" is the heavenly food that the Jewish Scriptures say that the Israelites ate in the wilderness after they escaped from slavery. But why, "dew"? Why is the manna in liquid form? It seems likely that the answer is connected with the rest of the complicated system of metaphor around water and dew in this poem.