Ibiza’s jellyfish are mostly the mauve stinger (Pelagia noctiluca): small, pinkish-purple and mostly see-through, with long trailing tentacles. They’re just drifting with the current, so getting stung is really about swimming into a patch at the wrong moment rather than anything to worry about beforehand.
They’re most likely from around May to October, when the water’s warm, and turn up wherever currents happen to push them close to shore that week: some coves, Sol d’en Serra is a known one, see jellyfish in season, along with a few spots on the west coast. Numbers shift year to year and week to week, so ask at a nearby bar or chiringuito before swimming in, since locals usually know what’s been around; the free “Medusas Ibiza” app also has crowd-sourced sighting reports if you want a heads-up before you head down to the beach.
If you do get stung, get out of the water, and don’t rub the area or rinse it with fresh water, since both can trigger more stinging cells. Rinse with seawater instead, and pick off any stuck tentacles with tweezers or the edge of a card rather than your bare fingers. A hot shower or a warm compress, as hot as you can comfortably stand, usually eases the pain better than ice. Most stings are just an itchy, burning welt that settles down within a few hours to a couple of days. Head to a pharmacy or the nearest health centre if the pain doesn’t ease, if a large area is affected, or if you get a wider reaction such as difficulty breathing, dizziness or swelling of the face: that’s rare, but worth treating as urgent.