Apparently they will sting you, especially if you make a noise akin to a Plankton. But only a little bit, as Plankton are so small that they require very little stunning. As the Jelly's sting is Alkaline, the cure is to be found within the neutralising effect that the acidity of vinegar offers. Plus hot water.
If you have a M.O.B. (Man Over Board) who comes face to face with one of these brainless beauties, you may well not have a cruet set with you, I believe their presence is in contravention of class regs. So... back to the bladder related cure as wee leans toward the acidic. More so if you like a lot of vinegar on yer chips. Although this is poo-poo'd in many quarters, apparently urea helps to dissolve the stuff that causes the stings to stick to your skin.
Or 'Any Port In A Storm'.In 1988, the entire bay was peppered with literally thousands of fried egg/flying saucer shaped, two foot diameter, silver grey, firm as a punch bag Jellys that I observed from a parakite whilst being towed behind a ski boat at 100 feet. Floating just below the surface rendered them undetectable from the shore. They must have weighed 30 pounds, as I tried to refloat one with a spade that washed ashore.
I frequently sight the smaller Jelly's about the size of a Softball throughout the summer months, so make a full bladder part of your boat's inventory.