This individual was ~9 cm across, but most were smaller in diameter. This is Aequorea sp., sometimes known as the Crystal Jelly.
Note its transparency and the hundreds of radial canals projecting outward towards the bell margin (see below). The tentacles are missing, as is the mouth, probably because these medusae were tumbled in the surf. But it's still a decent example of this distinctive species.
Aequorea are famously bioluminescent. There's a nice description and images showing the bright green points of bioluminescence encircling the bell margin on the Bioluminescence Web Page.