When I was a kid I was fishing off Daytona on a party boat, Al Kline's Snow White, and saw a Man O' War gas bag floating over my line. I reeled in and and a neon purple ribbon came up with the line, wrapped around the reel and, subsequently, my 12yo hand. The pain was intense as the mate poured vinegar over my hand. He explained the vinegar wouldn't stop the hurt - he was right!, but that it would keep it from getting worse. I never forgot that. I have encountered many jellyfish stings over the subsequent 35 years and always have a little vinegar on board.