One of the nice things about working right on the harbor is seeing the jellyfish come in during late summer.
Of course, I don't swim with them. And their poison won't freeze my face in a rictus of pain that prevents speech and spellcasting, like the death’s head jellyfish. Also, they are about the size of my fist, not 16 feet in diameter with a stored electrical charge that’s as deadly as a fireball, like the CR 11(!) sapphire jellyfish.
Moral of the story: If you are transported to a fantasy RPG setting, do not go swimming. Ever.
A tarnished bronze dragon specializes in robbing the ships of crusaders and knights templar. (As the knights templar are also the Central Sea’s default banking power, they are doubly affronted by the fallen wyrm’s brazenness.) Along the way, he has amassed a sizeable collection of religious artifacts that he keeps in a series of sea caves. These are guarded by a bloom of sapphire jellyfish. Not only is he immune to their electricity blasts, but any would-be treasure hunters are likely to expend their protection against electricity combating the Huge jellies, leaving them vulnerable to the dragon’s breath and the caves’ other traps.
Grindylows swarm up the locks at Savage Point. Swimming up the fish ladder meant to guide spawning salmon upriver, they are able to release nets full of death’s head jellyfish upon the helpless bargemen and lock tenders below.
A coral nixie challenges adventurers to a race. She does not intend to play fair, however. The route leads through a patch of death’s head jellyfish, past a sea anemone, and through a giant’s lobster traps (treat the giant lobsters as aquatic cave scorpions). Adventurers who sneak into her lair discover a room full of the grinning mummified heads of the jellies’ other victims. Some of these the nixie animated long ago as beheaded, using a now-inert magical rod (though it may have had other powers she could not unlock).
—Pathfinder Bestiary 3 155