The stone fish is the most poisonous fish in the sea and one of the most dangerous in the world. It can easily kill you if you step on it, injecting its venom deep inside your foot. If not treated promptly, the poison will kill you. This video by SmarterEveryDay shows how this simple killing machine works.
If you go to Australia, watch out for these bastards. They would be underwater, hiding under the sand or camouflaged over rocks, but also outside water, where they can survive for 24 hours. Apparently, stonefish antivenom is the second second-most administered in Australia.