A lake indicates that you have a deep spiritual reservoir for which you need to find an outlet. Water represents spirituality, and lakes develop where water flow is restricted. Spirituality is expression of your soul and in dreams is not connected with religion. Other symbols in the dream will tell you what spiritual aspect is being focused on. You are being asked to create an outlet for this in your life.
A polluted lake is about pollutants in your blood stream. You need to discover what substance is triggering these dreams and cut it down or cut it out.