I spend as much free time as I can get water-skiing from Cala Llenya up to Beniras, As mentioned it depends on the tides/currents/wind, You can have loads in one bay and none in the next some days, then not a one seen all the way up some day`s,
I have been stung a few times, It affects everyone different, But I do not find it much worse than stinging nettles although I have seen some with a mark still showing 3 months later and the pain they describe is not what I have had, If you are worried, go into one of the chemist and get yourself a "bite stick" that mostly stops it hurting straight away, If you are on a "life-guarded" beach, they will have treatment available.
Have a good one
Tim