Purple-striped jelly, Chrysaora colorata
I've been wanting to see one of these "in the wild" for, well, forever. The monterey bay aquarium usually has small ones in their jelly display, but they get so much larger in the ocean. The bell on this one was around a foot in diameter.
When I came across the jelly, I was at the end of my dive, and so was low on air. Also, my strobe was still set up for shooting tiny things inches in front of my camera. It took me a while for me to adjust my light, but the jelly wasn't going anywhere fast, and was only about 10' below the surface. So I had plenty of time, and didn't need to worry (much) about breathing my tank dry (which I didn't end up doing.)