In Preparation for Summer

Because of the wide variation in response to jellyfish stings, it is wisest not to contact jellyfish with bare skin. Even beached and dying jellyfish can still sting, when touched.

But:

Rescuers should wear barrier clothing, such as panty hose, wet suits or full-body sting-proof suits. Salt water may also be used if vinegar is unavailable.

Do not use fresh water if the sting occurred in salt water, because a change in tonicity can release additional venom. Avoid rubbing the wound, or using alcohol, spirits, ammonia, or urine which also encourage the release of venom.

Via Lifehacker