The good news is that not all jellyfishes produce painful stinging, and most of those who do live in tropical waters (especially in the South Pacific Ocean and around Australia). Unfortunately, among the latter, some may be really dangerous and may even lead to death because of the venemon itself, of allergic reactions or because the patient is in the water and may be unable to keep swimming.
Luckilly, European waters host just a few "stinging" species, usually not life-threatening. Among these, the small Pelagia noctiluca may be common along several European beaches. Should you touch it get out of water and call for help from baywatchers. They will be able to administer appropriate treatment and monitor the situation. In fact, allergic reactions in sensitive patients shall never be a priori ruled out.
As a general treatment, never rinse with fresh water as it may worsen the situation. Instead, rinse with sterile saline solution (if not at hand you may use sea water) and apply diluted acetic solution (white vinegar) or specific pharma-solution for 15-30 minutes. This will inactivate the toxin. Afterwards, carefully remove any remaining tentacle (use forceps!). Do not apply pressure immobilisation bandages, as this might induce further release of toxins from unexplosed stinging cells.
Stinging by P. noctiluca is surely an annoying accident that may occur when looking for relaxation in the summertime. Nevertheless do not worry: in the vast majority of cases it is only painful, leaving but a few superficial burns that will soon disappear!
For further reading (generally referring to tropical jellyfishes!!) see also:
The research leading to these results has received funding from the European Community´s Seventh Framework Programme (FP7/2007-2013) under Grant agreement n. 217766 