Giant Oarfish, Pig Butt Worms, and Other Freaky Creatures From the Deep

Every mermaid should know about these deep-sea creatures.

http://www.wired.com/wiredscience/2013/10/strange-deep-sea-creatures/

16th century mermaid

From an article on sea monsters from Wired.

This woodcut from 1562 shows that mermaids were already associated with a comb or brush and mirror. They either lured sailors to their doom or foretold it. They are probably depicted as oversize here in order to show the details. Notice the snake-like fishtails. No knees here.

There is a sea chanty (actually a sea ballad) about the crew of a ship preparing for death after sighting a mermaid. The Hard Tackers perform a version of it here.