This is a Portuguese man-of-war.
The man of war is actually made from a colony of organisms that work together. The tentacles can extend 165 feet or 50 meters below the surface of the water. The average however is 30 feet.
The man-of-war comprises four separate polyps. It gets its name from the uppermost polyp, a gas-filled bladder, or pneumatophore, which sits above the water and somewhat resembles an old warship at full sail.
Learn Something Fun: Man-of-wars are also known as bluebottles for the purple-blue color of their pneumatophores.