Reefs are valuable real estate, so giant moray eels don’t take kindly to intruders. And electric eels are…yes, of course, shocking.
Disarming a trap in a wizard’s lair saves adventurers from a watery doom. It also sets off a magical surge in the next room. The surge manages to both trigger the decanter of endless water that was a component of the trap and kill an apprentice doing lab work. His falling body breaks a tank containing electric eels, spilling them into the deepening cascade of water. The party must now ford past the excited eels to shut off the decanter.
When their skiff capsizes, a party of explorers finds themselves in the midst of a death’s head jellyfish swarm. A short half-swim/half-clamber will have them on the lee side of the reef, safe from the jellyfish but alarming a pair of giant moray eels.
A jungle temple complex turns out to be flooded up to waist-height thanks to the monsoon season. This means negotiating tlachtli courtyards filled with zombie ballplayers and electric eels. River drakes come to feed on both, and they interpret any action taken by humanoids against their meals, even in self-defense, as theft.
—Pathfinder Bestiary 119
For me, electric eels were a childhood fascination—they always showed up in cartoons—but somehow I forgot about them as I grew up (in a way I never did with, say, sharks and lampreys). Maybe it was the same with you?
I have a feeling the Hollow World inspired my zombie tlatchtli players.
Meanwhile, this article at Merriam-Webster is definitely GM-friendly.