Peeing on jellyfish stings doesn't help. It ooften sets off the sts and cause more venom into he area. What you do is do not rinse with freshwater either. you're dealing with still living jellyfish ells that will pump more toxin hen the fluid/salt balancee changes.
Instead use 5% solution white vinegar and beach sand on the affected site. the sand has trace salts and the vinegar neutralizes the toxin. The second thing to possibly do is baking soda mixed with seawater. The thrid thing to do is to take your credit card and rub it edgewise, scraping the affected area. If a human is stung, you can use a bic razor and shave the affected area to get those cnidocysts out of there. You can't really shave your dog, but the credit card works. and you can l always scrape with the credit card and then apply.