Everything I know of that urine works for, warm water works just as well if not better. The only reason people have ever used urine for it is due to warm water being more difficult to quickly create on-demand until the 20th century. Meanwhile, the inside of your bladder is always 98F/37C.

But for jellyfish stings specifically… No, human urine isn't hot enough to have the intended effect, the water has to be in a range of 104F/40C-113F/45C in order to denature the venom. Additionally, urine can exacerbate the effects of the venom the same way cool freshwater can.

That doesn't actually work, does it?