Australian Box Jellyfish
The Australian box jellyfish AKA Deadly Sea Wasp (Chironex Fleckeri) is the most venomous marine animal. The stings are extremely painful, and are often fatal to humans, causing death within 2 to 5 minutes. Fully grown, they can measure 8 inches long with 9 to 10 foot tentacles. They are commonly found in the coastal waters off North Australia, which includes the Great Barrier Reef.
Many swimmers wear lycra full body suits for protection. Australian lifeguards used to wear their wives pantyhose to protect them from the stingers, before the lycra suits. It was quite humorous to see these big bronzed Aussie lifeguards wearing pantyhose.