In my 4 years between St George and Arillas Ive never seen a jellyfish... although on my first year I did get some sort of sting/rash/thing from St George... it lasted a few months and can still see where it happened, not painful just irritating.
If you dont have a buddy to pee on you (Ive got a lot in England who'd offer) then Vassilles Market (spelling? Sorry..) sells an orangy/yellow mosquito stick which is all in greek. It happens to be the cheapest thing for bites that he sells at 90cents. It didnt do too much to the bites but the main mix is made up of Ammonia, which I reckon would look a lot better than having to pee on yourself.
And as for things to watch for in the sea, I have on 2 occasions had a fish hook in the foot/toe. Thats usually if you get closer to the end of the pier, but has never hurt too much.