Dreams involving water, such as lakes, often symbolize emotions and the unconscious mind. A deep, dark, foggy lake suggests that there may be unresolved feelings or situations that you're struggling to understand. The fog indicates confusion or uncertainty about these emotions.

Your ex-girlfriend's presence and her floating away on a small boat could represent feelings of loss or a longing for connection. The fact that she is not responding to you may signify unresolved issues or feelings about the relationship that you have yet to confront. The lantern she carries could symbolize hope or guidance, but in this context, it also highlights the distance and separation between you.

The mixture of peace and sadness you felt in the dream suggests an internal conflict. While there may be acceptance of the past or a sense of closure, the sadness reflects lingering feelings that may need attention. This dream could be prompting you to explore your feelings about the relationship and your current emotional state.

Consider reflecting on what this dream means for you personally. It might be helpful to think about what emotions or memories are being brought to the surface and how you can address them in your waking life. Journaling or talking to someone about these feelings could be a beneficial step in processing them.