Actually, there is alot of truth and some precautions in the advice so far.
Yes, urine is not the best, however, urine is diluted ammonia so it would work.
Alcohol is also good as it will help prevent additional stinging (remember that jellyfish stinging cells, nematocyts, can keep firing even after the tentacles are detached from the jelly).
Meat tenderizer....yes and NO. The key ingredient that helps is papain. Many meat tenderizers don't have it and will do nothing for you. Some brands do (Adolf's).
Jelly lotions...check the ingredients. Most will have papain or some other derivitive and work just like the proper tenderizer.