When stung by a jellyfish, some of the tentacles may stick to your skin. Brush them off carefully with a stick or several thick sheets of paper. Do not touch them! Urinate on the affected area. As an alternative, pour beer, soda, or vinegar, or dilute ammonia onto it.The venom is a protein, most proteins only function inside of a certain pH range, so a weak acid or base can deactivate them. If the sting is from a Portugese man-of-war, do not pour anything acidic on it because that supposedly activates the stinging cells.
Just as some people are allergic to bees, others are allergic to jellyfish venom, they may go into anaphylactic shock after being stung. In this case, monitor their breathing carefully and perform rescue breathing if necessary. If they carry an epinephrine autoinjector, use it. Get them to a hospital immediately!
The box jellyfish and some other varieties contain a venom that can be fatal in as little as two minutes. The anti-venom may be available at lifeguard stations. If you are at a beach with a lifeguard, notify them immediately. If not, be prepared to perform rescue breathing and get the victim to a hospital immediately. Be extremely careful not to touch the affected area. Or any tentacles that may have fallen off.