Well, there are two ways in which you could classify the deadliest animal in the ocean. One could interpret this to mean that the largest, most ferocious animal with the largest teeth would be most dangerous, and thus the answer of course would be the Great White Shark.
However, if you mean the most toxic, this could relate to a variety of invertebrates including the bite of Blue Ring Octopus or the sting of the Sea Wasp box jelly, both of which are from Australia. Both of these can cause respiratory arrest and death within only a few minutes from the time of initial exposure.

It depends on how you define "deadliest".Some of the most venemous animals in the sea include the Box jellyfish, Olive sea snake, and Blue ringed octopus. All these animals have the potential to kill a third grader, although you would have to annoy them a lot before they bite.
Anyone who has seen "Jaws" knows the capacity of a Great White Shark to kill humans, but more sharks attacks are reported for the Tiger Shark than any other marine animal.


Although we aren't technically marine animals, man has pushed to extinction hundreds of marine animals from small invertebrates like green abalone to right whales. We do this by catching animals faster than they can reproduce, and by fishing in ways that are destructive to the environment, like using dynamite on coral reefs or dragging a trawl on the sea floor.