Pools and lakes, as common places of purification or initiation, can represent a change in your way of life; emerging unconscious material that will promote growth. Like all bodies of water, it can represent the womb; pregnancy; a desire or need to return to mothering waters and be nourished or escape responsibility; knowledge that is deep or mysterious; possibly occult or metaphysical; your inner world of feelings or emotions.

If the surface is mirror like, it may symbolize reflection that leads to awareness. If a constructed like or pond, it may refer to the personal unconscious rather than the collective unconscious, often symbolized by the ocean or sea.
See boat, mirror, water.