This is the velella, a small free floating hydrozoan. It's currently the only known species in the genus.

also known as the sea raft or the by-the-wind sailor, the velella is a small species of free floating hydrozoan found in open waters worldwide. While they are not a siphonophore like the Man o’ War they live a similar lifestyle, in that they live on the ocean surface and use a sail for locomotion, and wait for food to move into their tentacles. also like most cnidarians they fall prey to nudibranchs like the blue dragon. They are found mostly on beaches due to the fact that they rely solely on their sail for locomotion, and some times that leads them to their demise.