If the offending jellyfish lives in nontropical waters, treat a sting by using seawater to wash the area and deactivate the stinging cells left behind. For stings in tropical waters, WebMD recommends using vinegar as a rinse before seeking out medical help immediately.
If water is used to wash the area affected by the sting of a tropical jellyfish or box jellyfish, it can reactivate stinging cells left behind, according to WebMD. Since the allergic reaction from a box jellyfish can be quite severe, it is important to seek out treatment from the nearest emergency department. Antivenom may need to be administered in the event of a severe reaction to the sting of the jellyfish.