Actually this is commonly thought of as a Jellyfish. Unlike a Jellyfish, which is a single organism, this animal is made up of a colony of organisms working together.
Divers, swimmers, and beach walkers should avoid any contact as they can give VERY painful stings, even when they are dead on the beach.
In the US they can be found in the Gulf of Mexico, and up the east coast of Florida.
Size: Float, 12 in (30 cm) long, 5 in (12.7 cm) wide; tentacles, up to 165 ft (50 m) long