Actually it's a man o' war which technically isn't a jellyfish.
Though the sail doesn't look much like a man o' war jellyfish. Blah.
I saw this one near the beach in Fort Walton and this little kid got wrapped up in one in the water. >:
It was as huge as a boat paddle, that's also what they carried it with when he came back on the beach.
Lemon Jellyfish [link]
Watermelon Jellyfish [link]