jellyfish stings

a blue blubber jellyfish awash on a beach — A photo we took of a blue blubber jellyfish on a beach in Melbourne, Australia in 2013. We saw them while swimming, we didn't know whether or not they could sting but avoided them out of caution, it was a good decision because we later learned that they do.

Stings from marine life can range from painful, to life-threatening.

If stung by a jellyfish:

Flush the injured area with vinegar for at least 30 seconds.
Remove any pieces of the animal while protecting your hands.
Apply heat (45°C|113°F) by immersing the limb in hot water or applying a hot pack.

Seek medical attention if there is an infection, or if the person has trouble breathing. Always flush with vinegar FIRST, before removing any part of the animal, the tentacles can continue to fire even after they're detached so you must neutralize the poison before touching them or they could inject more poison into the person.

Do nots:

Many older first-aid or medical books offer erroneous information on how to treat jellyfish stings, many online resources also have out-of-date information.

Do not scrape tentacles away with a credit card or blade, exerting pressure on the tentacles will result in them firing more poison into you.
Do not apply ice to a jellyfish sting, heat is a better option.
Do not pour alcohol, urine or freshwater/seawater to flush the affected area, it will make the pain worse, and may cause the tentacles to slide over a new spot while continuing to fire.
Do not apply baking soda and water pastes, or shaving cream, it won't help.

Source: Cubozoan Sting-Site Seawater Rinse, Scraping, and Ice Can Increase Venom Load: Upending Current First Aid Recommendations, by Angel Anne Yanagihara and Christie L. Wilcox. Published on 15 March 2017.