It seems it would depend on which story you read. However, based on the evidence, you would need to be very careful if you ever encountered one.
In British folklore they can be bringers of bad fortune, capable of causing storms and killing humans.
Some of the bad things that mermaids are accused of include telling sailors their ship is doomed and enchanting sailors and causing shipwrecks. Seeing a mermaid is a sure sign of a violent storm to come. In other stories, they deliberately drag people down in the water and squeeze the life out of drowning men. They also take men down to their underwater kingdoms.
However, on occasions, mermaids can also bring good fortune by giving humans cures or granting them wishes. In some tales, they even marry and live with humans. For example, the Merrow of Ireland and Scotland.
These mermaids are beautiful, gentle, modest and kind. They wear a red cap and, if this is captured and hidden from them, they will shed their skins and stay on land. However, most times they eventually retrieve the cap and return to the sea. They also lure young men to follow them beneath the waves. Here they live in an enchanted state. Merrow music is often heard coming from beneath the waves.
