A lifeguard told me I was stung. I was about 12 years old I think, in the ocean in Florida, and suddenly very painful on my knee. The life guard said there were a lot of man-o-war in the ocean that day. Pain lasted a long time. I remember it as being hours until it let up. I don’t remember if it lasted into the next day. If it did it must have been milder by then.
As an adult I’ve twice seen beaches with tons of blue jelly fish beached on the sand. I think the blue is the man-o-war. Sometimes the tides come in and they wash up I guess.
I also once saw lots and lots of clear jelly fish in a marina area, I don’t remember which state/country I was in. It made a impression on me though.
One time after swimming in the ocean I was incredibly itchy for days, and a bit of a rash, and I’m pretty sure it was sea lice, which is jellyfish larvae. It was sea lice season. Now, I know better, and always rinse off after being in the ocean. I’m very reluctant to go in the ocean during sea lice season though, I’d rather not risk. I think a lot of people who don’t know chalk it up to heat rash or sun rash, but it’s usually where your bathing suit is. Like if you have on a one piece you might get it on your stomach or side where the suit covers you.