Chronos said:
They will remind you when you tread barefoot on their tentacles.
I remember several "run-ins" with aquatic invertebrates of this type and others as a high-schooler in Hawaii. Once we took a weekend vacation at a locale which I think was close to the famed "black sands" beach on the east coast of Oahu. If I remember correctly, it may even have been Labor day!

1984, though

. In any case, I think I was smoking "pakalolo" on the beach at the time when a surfer buddy of mine came running up with his face turned white and his ankle swollen up like a red tree log telling me that he got attacked by these things called Portuguese "man-o-wars." Their bodies were really small, about the size of an orange, but they had this one long tentacle that extended out what seemed to be about 10 meters or so. And they had these really creepy blue stingers in them. It was nasty. There were thousands of them everywhere. Bummed my trip that weekend.
There was this other time, probably a couple years earlier, like 1982, when I first moved to Hawaii, when I was surfing on the tourist beach of Waikiki. These things called "reefs" were everywhere, basically coral growths in the surf break sands. Nobody paid much attention to them, you just had to put up with them. I think this was before that company put out those "reefwalker booties." In any case, I remember one day walking on one of the reefs after being caught inside after catching a wave. I felt this sharp pain scream up my left foot and leg. What happened is that I stepped on a sea urchin and it injected these poisonous spines in my foot. I remember hobbling, after great effort, down the beach to one of the lifeguard towers and informing them that I was going to die before I lost my virginity, hoping that might get me medical attention more quickly.
The lifeguard just kind of laughed and said I stepped on what they called, then, a "vana." for lack of a better cladistic definition. What he recommended to me was the same thing he recommended to all the the tourists that met that same fate; walk into one of the public restrooms on the beach and
urinate on your foot. So that's what I did. I remember standing at one of the urinals with my foot in the urinal and urinating on it while some fat old tourist guy from Nebraska in a g-string took the urinal next to me and watched me with mild dis-interest. People urinating on their foot in Waikiki in 1982 was not an uncommon or particularly shocking sight in public restrooms at that time.