Oh the contrary, I believe that mermaids are very much real. Most sightings by sailors were probably normal marine creatures, such as manatees, dugongs, or sea-cows (now extinct). These appear to cradle their young much as a human would carry a baby. It is possible that sailors, seeing these unfamiliar beasts, would assume that they had stumbled across some sort of humanoid species. Mermaids described by the famous explorer, Christopher Columbus, were almost certainly manatees. He reported seeing three mermaids in the ocean off Haiti, in January of 1493. He said they "came quite high out of the water", but were "not as pretty as they are depicted, for somehow in the face they look like men."
However such descriptions are very different from the usual portrayal of a mermaid and the idea, of a beautiful but dangerous creature, probably arose from the earlier stories of the Sirens of the Aegean Sea. The Sirens were sea-nymphs who had the power to charm by their song all who heard them, so that the unhappy mariners were irresistibly impelled to cast themselves into the sea to their destruction. The Sirens were first mentioned in Homer's Odyssey. This may have shaped the portrayal of mermaids from medieval times onwards. The modern view of mermaids has also been influenced by Hans Christian Andersen's fairytale, The Little Mermaid, written in 1836.
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.