Vinegar and a credit card are your best beach weapons.

Yes. For jellyfish stings, rinse with seawater (not freshwater), then pour vinegar over the sting to neutralize remaining nematocysts. If you don’t have vinegar, scrape the tentacles off with the edge of a credit card, shell, or sand-covered towel – never rub with bare hands. For sea lice (tiny jellyfish larvae), just rinse with seawater and change into dry clothes as soon as you’re out of the water. Avoid fresh water and alcohol; they can make things worse.

The real trick is knowing what not to do: no peeing on it, no rubbing with sand, no freshwater rinsing. Stick to seawater and mechanical removal. Pack a travel-size vinegar bottle next time – it’s worth the extra oz.