Sorry this is a bit of a long one and intended more for information than anything.
My half brother is a lifeguard and regular surfer in Cornwall and has been helping to deal with a lot of jellyfish stings and also panicing people as they leave the sea because of them. He wanted me to tell you something about them. as he thought the information may be useful for some as you head off on holiday.
Apparently there are a lot more jellyfish in UK coastal waters than normal because of the heat and also over fishing which means larger fish aren't eating them further out.
You may see barrel jellyfish They look terrifying but are apparently almost harmless UNLESS they catch you on an open cut :(
There there are compass jellyfish They are no worse than wasp sting - wash them in seawater and douse them in vinegar before running hot water over them for a while. Its also worth pulling a credit card or similar over the sting to free any bits of tentacle which may have got stuck in your skin.
There are also [[https://www.irishmirror.ie/news/irish-news/health-news/jellyfish-sting-lions-mane-irish-12883253 lions mane]] These give quite a nasty sting which some people have described as feeling a bit like an electric shock. Its probably worth getting out of the water if you see these as they have longer tentacles than many jelly fish. This is one it may be worth getting hospital treatment for if the pain does not subside fairly quickly.
moon jellyfish which don't sting and blue which I think do but are rare - you'll also notice them pretty quick because they are well blue!
Finally if you see a Portugese man of war please leave the water, alert those around you to do the same and tell life guards if there are any on the beach. If you get stung seek medical treatment. HOWEVER these are very rare in UK waters, half brother has never seen one but he likes people to know.
He says have fun, watch for rip currents (the calmer the sea looks the more likely there is one but ask locals if you're not sure), don't swim when the red flag is flying and if you've got small children in inflatables please hold on to them as he had to pull a number of children out of the sea last year and call a life boat for one as well.