The lake in dreams is associated with the love and affection. The lake that seems very calm and still, indicates very stable relationships with your lover. If the lake is a bit wavy, or there is some rain pouring on it, then you will face a hard times, especially in your personal love affairs. The dream in which you see some kind of the beast in the lake foretells that you are afraid of unknown.
Read more about dreaming of Lake in other dream meanings interpretations.