According to myth, they lure sailors by singing and with lovely music. They live in a kingdom on the bottom of the sea, and it is here they take their prisoners to. From this story, the fear amongst the sailor grew and they thought that seeing a mermaid would cause bad luck: it could predict death by drowning.
The belief in mermaids is not limited to a few countries, but there are tales from all over the world (in India, for instance, there are the Apsara, beautiful water nymphs). However, most of those tales were told by sailors who "saw" them on their long journeys. The idea of mermaids and mermen, the male equivalent, could be based on creatures from Greek and Babylonian mythology: Sirens and Tritons of the Greeks, and the fish gods, who were half human and half fish, from the Babylonians.