Jellyzappers are a species, or perhaps multiple species, of mutant jellyfish. They're most notable for their method of locomotion and hunting: filling their translucent bodies with a mixture of lighter-than-air gasses that allows them to emerge from the water and float in the air for hours at a time. They typically do this at night, so as to draw insects into their tentacles, attracting them with their bio-luminescent bodies. Singularly, their venom is not life threatening to adult humans, but they often congregate in large numbers. The lazily floating herds make little noise and can drift over a boat with ease if passengers are not vigilant.
Jellyzappers have been spotted off the Gold Coast, throughout the Gulf of Mexico (especially drawn to the mass graves of the Corpse Coast) and even as far west as the Sea of Cortez. They've not been seen further north than that, but it's possible that they range further south.