Phoenix_Rising and Dracula have it spot on. . .

I was stung across the face while scuba diving and it really does feel like a combination between electrocution and acid burn. It hurts a LOT.

Vinegar and hot water are the two methods that divers use; we generally carry vinegar in the boat with us as a first method of treatment. We use very hot water after that - it needs to be so hot that you can barely stand it, but not hot enough that it will burn/scald you.

Hopefully you won't get stung, though :(