Porpita porpita, the Blue button;
Volusia county, Florida (19 May 2014).
Learn more about this species at iNaturalist.org.
Not actually a jellyfish, the Blue button is better described as a colony of hydrozoan polyps functioning together as one organism. In this shot, you can see the dactylozoids radiating outward from the central float (disc). Each of those dactylozoids carries with it a series of stinging nematocysts, but the sting doesn’t pose much of a threat to people. The is a dorsal top-down view of the Blue button.