Shiff arms jellyfish (Rhizostoma pulmo)
Medes, Spain. In some spots there were a couple of these. This picture was taken in front of the cave where hundreds of jellies were residing. Called Borm blau in the Catalan language.
This is a non-stinging harmless jellyfish, that feeds on plankton. I was afraid of it at the time, but that is not neccessary with this species.
Apparently this the most interesting picture on Flickr tagged with Scuba (click on little globe next to a tag and sort by "most interesting). I am flattered.