Sirens have been depicted as part-woman and part-fish. When they sense a ship or crew coming,sing a song so soothing that it makes the sailors do whatever the song says, which is usually wreck their ship on jagged rocks. It is unknown what the Sirens do with sailors they capture though most believe they're eaten. Their beauty and song are used to lure the weary sailors into a sense of security before showing their true form, and intentions.
To avoid the "Siren's Song", you can plug your ears with cotton or other material, so you cannot hear anything. If the captain is brave enough, he will have his crewmates tie him to a mast, and he will endure the song until it is over(as Odysseus/Ulysses did).