About 6 years ago I was living in Australia, had a Hungarian expat friend with a NICE boat, nothing too fancy but a good diving rig. We went out initially intending to go spearfishing but couldn't find any food fish worth the trouble, so we decided to get baked and drive the boat out to a nearby reef instead. Parked the boat, blaaaaaazed, maybe twenty minutes later I decided to go snorkeling on the reef...
Well it turned out that I was RIGHT in the spot where a large group of (harmless) cannonball jellyfish were "swarming", hundreds of them. It's weird enough to go swimming in the warm West Australian ocean and be surrounded by sea critters, it's another thing to be stoned out of your gourd while doing it, and the whole time little wriggly balls of jello are bumping into you and poking you with tentacles...