They def sting. This dude about a month ago duck dived and came up under a bunch of the. Shoulda seen his face and eyelids and neck. Major welts and pain. Cold clear water will cause the tiny pieces in your skin to release more toxin. Use vinegar. There is an anti sting lotion thats called SafeSea Jellyfish sting protective Lotion. I used it: my buddies all got stings, I didn't. So maybe it works. It creates a chemical barrier. Can get it online.
The SafeSea lotion works good but it is thick, gooey and expensive. I use it when the sea lice are bad. I got stung in the eye by a moon jelly Sunday duckdiving. They were really bad a week or so ago, but the sting wears off in about 5 minutes or so. We used to throw them at each other when we were groms. Just scoop one up and throw it on your bros back as he scans the horizon for the next set. Splat/Yow!