A practical guide to the natural inhabitants of the sea that can prove dangerous to swimmers, divers and other humans working or enjoying themselves within a marine environment. The text surveys all the hazards, from sunburn and malaria, to the ornate wobbegong (best left alone when sleeping) and the fine-spotted porcupinefish (deadly to eat). All these "terrors of the deep" are illustrated in colour photographs with accompanying symbols indicating their degree of danger or toxicity, their habitats and their faunal zones throughout the world. The photographs are all taken from the "Australasian Marine Photographic Index", but the references are often to creatures that can be found anywhere in the sub-temperate, sub-tropical and tropical areas. Humans are not the normal, natural prey of marine creatures, yet in the past we have considered ourselves (due to irresponsible media exposure) the menu of every creature in nature. The concept of this manual is to help humanity recognize the actual dangers and deal with them in a responsible way.