El Monstruo del Pantano (Estigma de Muerte) (1965)

You are Karen.  You’ve been kidnapped by your father’s mildly disfigured assistant Egon.  Egon can transform into a very poorly designed man/jellyfish hybrid monster, and has killed off all your nubile friends for making fun of him.  For the disfigurement, not the whole crappy were-jellyfish thing.  FIght your way out of the undersea lair of this underwhelming villain, with help from the ghosts of your dead girlfriends, and your boyfriend John.  Featuring the Neil Sedaka smash hit ‘Do the Jellyfish’!