In general, water represents the subconscious. Diving into it symbolizes our insertion into the most hidden parts of our mind. On the other hand, the dream could be encouraging us to be opportunists, and that you dive in regarding a particular issue. In many myths, the hero dives into the depths of the sea, and returns to the surface with a great treasure. In the case of dreams, this represents the self that ventures to explore the subconscious and finds the valuable and true self.