Researchers in Australia, home of the deadliest species of box jellyfish, have spent years studying jellyfish stings. One thing is certain: nothing works for sure. Vinegar (5% acetic acid) has been shown to be the best rinse for box jellyfish. It neutralizes unfired nematocysts so they can’t inject venom. When vinegar is not available, most research suggests using sea water to rinse away the remaining nematocysts. Freshwater is no good; it triggers nematocysts to inject venom.
A word of caution about vinegar: Studies suggest that vinegar actually worsens the pain of Portuguese Man of War, bluebottle, and other Physalia stings. These creatures are dangerous look-alikes to jellyfish. Vinegar has been shown to cause nearly 30% of Physalia nematocysts to fire.