I live on the east coast of Florida and I've had a couple of encounters
with the Portuguese Man Of War. Though it looks like a jellyfish, it
isn't. It's far more complex as this video, uploaded by slk320k on September 21, 2009, will show...
Egad! I never realized that the tentacles hung up to 30 meters below the surface! Gah!
And a little bit of advice: if you happen to see one of these things washed up on the beach, stay away. Don't prod it with a stick, don't try to step on its gelatinous floating sack--just stay away.