In dreams, mermaids are strongly related to sexual power. If the dreamer is a man and during the dream he hears mermaids singing, its usually a warning about a woman thats trying to catch his attention with not-so-honest purposes. If the dreamer is a woman, it usually means she has doubts about her own femininity. Generally speaking, mermaids and mermen are subconscious creations, they usually symbolize desires that may lead us to our own destruction.
Read more about dreaming of Mermaids in other dream meanings interpretations.