Tony said:
They usually prefer being very close to shore - the water is warmer
If you are worried about jellyfish the best thing to do when you get to any beach in Ibiza or mainland Spain is to see how many spaniards are in the water....if there are none you can probably count on there being jellyfish on the shoreline.....
Another top tip if there are jellyfish is to go in the water where there are loads of kids....they are usually all competing to find the jellyfish and are usually pretty good at fetching them out of the water with there nets...