One of the deadliest creatures on the planet is the "Sea Wasp" Box Jelly (Chironex fleckeri) - a species that can kill a person in a few minutes. These jellies are found along the shallow coastal waters of Australia.
Box jellies are becoming more visible in Hawaii. Fortunately they are not the same species as the deadly Chironex fleckeri. Wakiki Aquarium has posted a very handy online calendar that provides the probability of box jellies on popular beaches based on the observation that they tend to appear between 8 to 10-days after a full moon.
Here's what the Waikïkï Aquarium and the University of Hawai‘i-Mänoa have to say about the species of box jellies found in Hawaii:
Box jellies (also known as jellyfish) belong to the invertebrate Phylum Cnidaria, a diverse group of stinging animals whose members all possess stinging cells for feeding and protection. Jelly/jellyfish relatives include the sea anemones, corals, and Portuguese man-of-war. The box jellies, or Cubomedusae, are named for the squarish shape of their bell-shaped body. Three species are now known in Hawaiian waters, Carybdea alata and a species Carybdea rastoni, and Carybdea Sivickisi. C. alata is the largest of the three, reaching sizes of one and a half to two inches and diameter and three inches in height. The smaller species is about a third the size. Each of these box jellies has four thin, pinkish tentacles that trail from the "corners" of the transparent body. Complex sensory structures are located between the tentacles, just above the bell margin; each contains a balance organ and a light detector that includes a lens. Box jellies are capable of directed swimming toward a light source, and are reported as the fastest swimmers among the jellies and their relatives. They are active predators, capturing small fishes and crustaceans with their potent sting.
As a group, box jellies are found in shallow tropical seas throughout the Pacific, Indian, and Atlantic Oceans. They generally occur in quiet, shallow waters of protected bays and estuaries, and over sandy-bottomed shorelines, though some species have been reported in the open ocean. Box jellies apparently descend to deeper water during daylight hours, but during summer months, adults are often reported at the surface. The tentacles, well-armed with potent stinging cells can inflict a painful sting on unwary beach goers. An Australian relative, the notorious "sea wasp" box jelly (Chironex fleckeri), is deadly. While the sting of Hawaii's box jellies is not usually lethal, it is reported to be more painful than that of the more common Portuguese man-of-war.
If you're planning a trip to Hawaii, the Hawaii Aquarium website is well-worth a visit!
