These days we think of mermaids as friendly and human-like. Disney's Little Mermaid acts just like any teen-age girl.
The mermaids in PoTC/OST are different. Grown men throw themselves into the sea rather than face the mermaids. It turns out that they have good reason for this behavior. With the exception of Barbossa and a small landing party, the entire crew is killed by the mermaids.
The mermaids of legend were not much better. They were not as direct but they were often sirens, luring ships into shallow waters where the ship would run aground and its crew would drown.
Other times they were simply a bad omen. Seeing a mermaid meant that you were in danger (possibly because they liked the shallows). Blackbeard seems to have believed this. According to Wikipedia:
The logbook of Blackbeard, an English pirate, records that he instructed his crew on several voyages to steer away from charted waters which he called "enchanted" for fear of merfolk or mermaids, which Blackbeard and members of his crew reported seeing.
Anything that Blackbeard was wary of was dangerous, indeed.
