I was in Fuengirola last week, only went in the sea once though. I didn't see any jellyfish and also there were quite a few people in the sea so it didn't seem to bother them.
I was stung by a small jellyfish last year. I saw what I thought was a clear plastic bag floating by me but felt a stinging on my arm and saw I had a burninging red mark about the size of my fist on my forearm. I went and lay down, thinking that I was going to be really ill or pass out or something as it was burning quite a lot but it gradually eased off over the next few hours and had gone by the next day.
Apparently it's usual for restaurants to have to supply some vinegar to put on the sting but human urine is another remedy. Another Spanish recommendation is to rub wet sand onto it. Perhaps you could take along a bottle of vinegar just in case someone is stung.
A couple of years ago, in Fuengirola again, there was a HUGE jellyfish washed up on the beach. I would describe it as looking like someone had emptied half a bucket of wallpaper paste onto the sand as it was about the same colour and was the size of a large dinner plate. I'm certainly glad I wasn't stung by one that size!