According to my understanding of the Wikipedia page on the stinging cells of the jellyfish, the mechanism of delivering the toxic missile is largely due to water pressure. Does that mean that a jellyfish on land (or brought out water) is unable to, or perhaps less efficient at, delivering its stings?
Jelly fish can sting out of the water and even when they are considered to be dead.
I do not have detailed scientific literature available. However, popular sources generally and equivocally warn against touching jelly fish, even on land and even when they appear to be dead. In fact, dead box jelly fish can be as dangerous as live ones (ABC News).