If you get stung by a jelly fish, don't ask your friend to pee on you. Similarly, reading in the dark won't make you go blind. For explanations and more interesting body myths, head over to Life Hacker.
Which makes me want to ask how on earth did the myth get started in the first place? I'd heard it in Australia way before Friends. There must be a lot of people out there with a very mean sense of humour.
My cousin was severely stung by a jellyfish on vacation once and she can attest to the fact that urine does not soothe the pain. She had two different men pee on her as she went into shock...