Mainly, seeing a mermaid or a siren was considered by sailors as a sign of bad luck, as they were thought to attract ships on rocks by the spell of their enchanted voices.
For many cultures anyway, sirens are a powerful symbol of fertility and sensuality. They are often depicted while combing their long flowing hair (a symbol of abundant love potential), and Venus, the Greek goddess of love and passion, was thought to be born from the foam of the sea. The Greek word for "comb" itself also signified the female vulva.