A Siren isn't really a sea monster but sailors have told tales of these beautiful women that sing to you and seduce you into their trap of bloodlust. They are said to eat the sailors alive or drowned them deep in the ocean.
Hades and Persephone. Interesting perspective from the artist because it makes Persephone appear as though she actually loves Hades. < They actually had a fairly happy relationship compared to the rest of the Greek gods (I'm looking at you, Zeus)