The tentacles of the sea nettle contain millions of microscopic stinging cells called nematocysts that inject toxins to capture and paralyze prey as well as to defend the jellyfish from would-be predators. When a swimmer brushes against a tentacle, the resulting sting is painful and annoying. There are several things that you can do if you get stung. If bits or pieces of tentacles are still on the skin, pour alcohol or baby powder on the area. Alcohol will stabilize the nematocyst so that it will not be triggered.
Powders do the same by drying the cells out.
Without such treatment, tentacles which are disturbed may release additional nematocysts, causing additional irritation and swelling. Next, apply diluted ammonia, sodium bicarbonate, vinegar or meat tenderizer to the area to relieve pain. Meat tenderizer is one of the best sources of relief from stings. Add a small amount of water to the meat tenderizer to make a paste and smear it on the inflamed area. Meat tenderizer is an enzyme which breaks down proteins.