The idea that all life came out of the water is very widespread, it also corresponds to today's theory of evolution.
The dream signals to the dreamer that he has access to the collective unconscious. C.G. Jung sees the fish as a symbol of the self. Jung thus means the totality of consciousness and subconsciousness. They are to be regarded positively as food and thus as an expression of psychic energy, but also threatening danger as great living beings which we can not overcome, because they suddenly come upon us from the unfathomable depths (of consciousness!). A dream about fish establishes the connection to the emotional side of the dreaming. He makes clear that the talent of the dreaming is not in strategic thinking. Often, however, an instinctive response is more appropriate than analyzing a situation. Since he lives in the water and the water is a symbol of the unconscious, the fish also points to the subconscious-driven impulses of the dreaming, who are too suppressed or rejected, or there is the desire for children behind it. Even in Babylonia, fish was considered a phallic symbol. This then points to the emotional side of sexuality. However, to interpret the fish sexually generally would be too one-sided. The ancient Egyptians saw in the fish the symbol of the soul. In Buddhism, fish is considered a sign of spiritual power.
In my view, they outline the depth of the human soul. Depending on the concomitant circumstances, different individual interpretations arise.