Jellyfish

Vinegar (3 to 10% aqueous acetic acid) is a common remedy to help with box jellyfish stings, but not the stings of the Portuguese Man o' War (which is not a true jellyfish, but a colony). For stings on or around the eyes, a towel dampened with vinegar is used to dab around the eyes, with care taken to avoid the eyeballs. Salt water is also used if vinegar is unavailable. Fresh water is not used if the sting occurs in salt water, as changes in tonicity can release additional venom. Rubbing wounds, or using alcohol, spirits, ammonia, or urine may have strongly negative effects as these can also encourage the release of venom.
Clearing the area of jelly, tentacles, and wetness further reduces nematocyst firing. Scraping the affected skin with a knife edge, safety razor, or credit card can remove remaining nematocysts.
Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) can control skin irritation (pruritus). For removal of venom in the skin, a paste of baking soda and water can be applied with a cloth covering on the sting. In some cases it is necessary to reapply paste every 15–20 minutes. Ice or fresh water should not be applied to the sting, as this may help the nematocysts to continue releasing their toxin.