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Sea snake -

Adapted to a fully aquatic life and unable to move on land, sea snakes are highly venomous. The most poisonous species is the beaked sea snake, whose venom is eight times as toxic as a cobra's.

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Box jellyfish -

There are at least 51 species of box jellyfish. Some produce extremely potent toxins and have earned the dubious honor of being among the most venomous creatures in the world.

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Blue-ringed octopus -

One of the most dazzling of mollusks is also recognized as one of the world's deadliest marine animals. Relatively docile in nature, they are dangerous to humans if provoked.

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Great white shark -

Steven Spielberg's 1975 movie 'Jaws' did the reputation of this iconic species no favors at all. The fact remains, however, that the great white shark is responsible for by far the largest number of recorded shark bite incidents on humans.

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Pufferfish -

The majority of pufferfish species are toxic and some are among the most poisonous vertebrates in the world. Yet the meat of some species is considered a delicacy in Japan.

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Stonefish -

All members of the stonefish family are toxic, dangerous, and even fatal to humans. In fact, they are the most venomous fish out there.

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Piranha -

The piranha has a fearsome reputation as having one of the strongest bites found in bony fishes. Rows of razor-sharp teeth in both jaws can shred prey to pieces in minutes.

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Lionfish -

Lionfish are known for their venomous fin rays, an uncommon feature among reef-dwelling fish along the American East Coast and Caribbean. Get stung by one of these guys and expect extreme pain, nausea, vomiting, fever, breathing difficulties, convulsions, and dizziness.

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Tiger shark -

Named for the dark stripes down its body, which resemble a tiger's pattern, this shark species is second only to the great white in recorded fatal attacks on humans.

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Saltwater crocodile -

Saltwater crocodiles are among the most active of all crocodilians, and this particular species is especially aggressive when humans unknowingly venture into its territory. In fact, the saltwater crocodile and Nile crocodile have the strongest tendencies to treat humans as prey.

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Nile crocodile -

An apex predator, the Nile crocodile is capable of taking almost any animal within its range, including us.

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Electric eel -

There are few documented instances of people dying from an electric eel's shock, but it does happen. A single jolt could incapacitate a person long enough to cause him or her to drown; multiple shocks can induce heart failure.

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Candiru -

The candiru is a species of parasitic freshwater catfish native to the Amazon Basin. The smaller species are known for an alleged tendency to enter the urethra of hapless swimmers and then lodging itself in place with sharp barbs. Next, it proceeds to feast on its victim from the inside—a horror story that is enough to keep your legs firmly crossed for days on end.

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Sting ray -

Generally passive in nature, a sting ray will attack humans only when provoked. Contact with the stinger in the tail causes local trauma (from the cut itself), pain, swelling, and muscle cramps from the venom.

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Cone snail -

Killer snails? You better believe it! Carnivorous and predatory, cone snails stun their prey by injecting peptide neurotoxins called conotoxins. Because all 600 or so species of cone snail are venomous and capable of "stinging" humans, live ones should never be handled, as their toxic discharge will occur without warning and can be fatal. Two are known to have killed people: the geography cone and the textile cone (pictured).

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Great barracuda -

A large, predatory fish, the barracuda has been known to attack humans if provoked. In fact, there are recorded instances of barracudas leaping out of water and injuring boaters.

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Hippopotamus -

A large, mostly herbivorous, semi-aquatic mammal, hippos are among the most dangerous animals in the world due to their highly aggressive and unpredictable nature. Small boats can easily be capsized by an angry hippo, and passengers can be injured or killed by the animal, or drown.

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Tigerfish -

Streamlined, muscular, and extremely aggressive and capable predators who often hunt in groups, tigerfish are armed with interlocking razor-sharp teeth that would do a piranha proud. Despite their size, the species is highly agile: the African tigerfish is the first freshwater fish recorded and confirmed to attack and catch birds in flight!

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Snakehead -

Snakeheads are predatory fish that possess an amazing trait: they breathe air with gills, which allows them to migrate short distances over land. While native to parts of Africa and Asia, the introduction of this species into the United States has alarmed environmentalists. Snakeheads are highly invasive and scientists fear the fish is able to spread parasites or diseases.

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Diving bell spider -

The diving bell spider is the only species of spider known to live almost entirely under water. They do so by building a diving bell of irregularly constructed sheets of silk which they then inflate with air brought down from the surface. Humans unlucky to disturb this marine arachnid can expect a painful bite that causes localized inflammation, vomiting, and slight feverishness that disappears within a few days.

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Anaconda -

The common or green anaconda is the largest snake in the world by weight, and the second longest, after the reticulated python. The anaconda's domain is the swamps, marshes, and slow-moving streams of the Amazon and Orinoco basins. While attacks on humans are very rare, encounters have taken place where people have been bitten and subsequently constricted before being rescued.

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Bristleworm -

Of the many species of worms, the bristleworm is the most dangerous. They occur throughout the Earth's oceans at all depths, and in a varying range of temperatures. None of them should be touched with bare hands. Their bristles are very thin and will embed in your skin, causing a severe itch. Pictured is a rag worm, a species of bristleworm.

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Weeverfish -

Many a unsuspecting beachgoer has fallen victim to the weeverfish. During the day, the fish bury themselves in sand in the shallows, just showing their eyes. Anybody unfortunate to step on one will undoubtedly receive a nasty sting inflicted by venomous spines on their dorsal fin and gills.

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Water moccasin -

The water moccasin, a species of pit viper, is the world's only semi-aquatic viper, and is native to the southeastern United States. As an adult, it is large and capable of delivering a painful and potentially fatal bite.

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Harmful algal bloom -

According to the National Oceanic and Atmospheric Administration (NOAA), harmful algal blooms "occur when colonies of algae—simple plants that live in the sea and freshwater—grow out of control and produce toxic or harmful effects on people, fish, shellfish, marine mammals and birds." NOAA points out that human illnesses caused by these blooms, though rare, can be debilitating or even fatal. Pictured is a NASA satellite image of Lake Erie in North America during an algal bloom in 2011.

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Needlefish -

This sleek and streamlined fish is not aggressive, venomous, or poisonous, and it doesn't even have a fatal bite. Instead, the danger lies in its ability to become airborne. Needlefish are known to launch themselves out of water at great speeds. Anybody in the way of one of these aquatic missiles runs the risk of being seriously pierced.

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Sea lion -

Don't be fooled by the sea lion's cute exterior. When agitated or threatened, these super agile creatures can launch a swift attack. Assaults on humans are rare, but get anywhere closer than 2.5 m (8 ft) to an adult sea lion and it can be very unsafe.

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Caribbean fire sponge -

There are approximately 5,000–10,000 known species of sponges found in the world's oceans. Most are harmless to humans but some, like the Caribbean fire sponge (pictured), cause a severe skin rash if handled.

Sources: (World Health Organization)(National Oceanic and Atmospheric Administration) 

See also: The world's most colorful reefs

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