Then there's this thing. Or, rather, this colony of many things.
The Portuguese Man o' War (Physalia physalis) is not a jellyfish. It is, instead, a colony of small hydra-like polyps. Each of these creatures has a different role to play: some become tentacles, which can get up to 66 ft long; others become the digestive system; one big huge polyp makes the sail. Imagine if your brain, spleen, stomach and liver were all different, highly-specialized individuals. That is how the Man o' War lives.
The sail - that thing that looks like an inflatable pool toy up there - keeps the Man o' War floating. Unlike jellyfish, Man o' Wars have no way of propelling themselves through the water. The sail can be deflated and inflated at will, allowing some Man o' Wars to remain hidden beneath the surface. Screw those fake shark fins; the presence of even one Man o' War can close entire beaches.
You will be assimilated...fabulously.
Where there is one Man o' War, there are usually others. Since they are forced to go wherever the wind blows them, there tend to be giant swarms of deadly water balloons with potentially over 1,000 individuals. Sightings of said swarms used to be limited to warm waters, but, in recent years, they have been spotted as far north as Ireland.
The Man o' War, while not actively out to kill you, lets its (their?) meter-long tentacles dangle continuously beneath its sail. These are usually used to poison small shrimp and fish, but are dangerous to humans was well. The sting is usually just painful as hell with some welts to make your mistake visible to the world. If one is allergic or venom travels to the lymph nodes, however, the stings can be lethal. Deaths are rare, but the presence of even one bubble floating above the water's surface can still be cause for alarm.
Oh, and they can still sting after death.
The news isn't all bad. The Man o' War carries with it some smaller marine fish that are found almost nowhere else. Species such as the clownfish seek Man o' War tentacles as protection; they themselves are immune to cnidarian venom (as Finding Nemo so nicely demonstrates). The smaller fish might lure in larger fish for the Man o' War to eat, so they're all cool with having Nemo in their tentacles.
"I wanna eat him." "NO!"
Many things also use the Man o' War for food, weaponry, or both. Loggerhead turtles eat the Man o' War for breakfast; Glaucus sea slugs not only eat the things, but use the cnidocytes as their own weaponry. The blanket octopus steals Man o' War tentacles instead of using ink; aren't you happy that Cthulhu exists NOW?
Even the Japanese agree that Lovecraft was right.
Why are people still terrified? Because not everybody's a loggerhead sea turtle, Glaucus slug, blanket octopus or another creature that finds Man o' Wars (Men o' War?) tasty...