Water is most often interpreted as the depths of emotions within the unconscious.  People are often drawn to water because it has a calming effect on them.  It also helps them to get in touch with deeper feelings and to better connect with their souls.  Swimming in a lake or ocean in your dream can mean that you are currently exploring the complexities of your thoughts and emotions.  Water in dreams can be either calming or terrifying.  See Flooding to read about the scary side of water in dreams.