The Portuguese man-of-war (sometimes "man-o'-war") is a fascinating organism. Its venomous tentacles are extremely painful but not generally deadly to humans.
According to animals.nationalgeographic.com, it "is . . . not a jellyfish, it's not even an 'it,' but a 'they.' The Portuguese man-of-war is a siphonophore, an animal made up of a colony of organisms working together." Its "float [is] 12 in (30 cm) long, 5 in (12.7 cm) wide; tentacles, up to 165 ft (50 m) long."