The Most Dangerous Creatures You’ll Find in Water

The world’s oceans, lakes and rivers host hundreds expeditions and races every year (except perhaps this one). But water is dangerous… strong currents, rip tides and hypothermia are obvious risks. But what are the more unusual aquatic threats?

Harmful Algal Blooms

Harmful algal blooms occur when colonies of these simple photosynthetic creatures grow out of control. Some produce toxins that are harmful or even fatal to humans. Generally, HABs grow in warm, slow-moving water with lots of sunlight and are often more severe in polluted waters.

Over the past decade, an increasing number of lakes closed to recreation because of HABs. In 2010, for example, local authorities shut Grand Lake St Mary’s, in western Ohio, after many people reported suffering stomach aches, rashes and numbness after swimming. One man, whose dog had died from HAB poisoning, struggled to walk, lost all feeling in his arms and legs and had to be hospitalized.

Lake Erie toxic algal bloom. Photo: NASA


The most common HAB is from blue-green algae, a form of cyanobacteria found near freshwater shorelines. Although most species aren’t harmful, a few produce cyanotoxins that cause rashes and allergic reactions in the eyes, ears, nose and throat. If the toxins are swallowed, they can lead to serious stomach, kidney and liver issues, as well as respiratory and neurological problems.

Freshwater threats

Most of us might think of piranhas and crocodiles as the most dangerous freshwater animals. While piranhas should be avoided, they are not the deadly man-eaters that Hollywood films make them out to be, as we recently explained. They mostly scavenge dead animals and will generally only attack a human as a last resort or if they feel threatened. The same cannot be said for crocodiles.

Nile crocodile. Photo: BBC Earth


Crocodiles are opportunistic predators for whom any warm-blooded mammal, including a human, is fair game. Africa experiences hundreds of crocodile attacks a year, and up to half of these are fatal. The danger depends on the species. In freshwater, the most dangerous is the Nile crocodile.

Researcher Simon Pooley — who grew up in Africa and whose father did pioneering studies on this species of crocodile — has studied 65 years of crocodile attacks in Africa. He says that attacks are seasonal and happen mostly between October and March. They attack more on sunny, warm days and are less active in the cold.

In one famous incident in 2010, 35-year-old Hendrik Coetzee was leading a three-man kayaking expedition on the Lukuga River, Congo when a Nile crocodile grabbed him from behind and pulled him underwater. He, Ben Stookesbury and Chris Korbulic were paddling close to each other in the same formation that they had used on the rest of the river, to give the impression to potential predators that they were a larger animal. Coetzee didn’t even have time to cry out before the crocodile had dragged him under. His body was never found. Coetzee was a well-known kayaker and had spent a decade recording first descents on the wildest rivers in Africa.

Including caimans and alligators, 8 of 23 crocodilian species have carried out unprovoked attacks on humans: the Saltwater Crocodile, Nile Crocodile, American Alligator, Black Caiman, Morelet’s Crocodile, Mugger Crocodile, American Crocodile and the Gharial.

Another animal favored by melodramatic Hollywood scriptwriters, the electric eel is actually more closely related to catfish than eels. It lives in the Amazon and Orinoco basins of South America and can produce pulses of electricity over 800 volts.

Researchers have discovered that the eels sometimes leap out of the water to channel their shock into whatever they are attacking. Intrigued, biologist Kenneth Catania set up metal plates to detect the eels’ electrical currents during these leaps. He found that the voltage increased with the height of the attack.

He then wanted to get a sense of the current delivered when they were attacking an animal, not a metal plate. So Catania made himself the guinea pig and allowed a small, foot-long eel to shock his arm multiple times. The current delivered was 40-50 milliamps when his arm was furthest from the water — 10 times as powerful as a taser. With that sort of power, they could easily harm animals and even humans, if the eels were larger. Human deaths from electric eels are rare, but multiple shocks have occasionally led to respiratory and heart failure. Some people have also drowned after being stunned by the shock.

Candiru. Photo:


The Amazon River is home to many creatures you would rather not swim into, and the harmless-seeming Candiru is the stuff of nightmares. These tiny, parasitic catfish typically feed on the gills of larger fish, but have been accused of occasionally lodging in the urethras of men and women, attracted by urine.

Although a few documented cases of Candiru attacks do exist, scientists remain skeptical: In experiments, the fish show no interest in urine, and with an average length of three inches and width of a quarter of an inch, they are really too large to enter a urethra. It seems that the reputations of these little fish are grander than their actual danger.

Diving Bell Spider. Photo:


One of the lesser-known freshwater dangers, the diving bell spider is the only known spider that lives completely underwater. It is a truly fascinating animal: As an arachnid, it needs to breathe air, so it forms its own air bubble which it holds in place with the hairs on its legs and abdomen. It surfaces about once a day to replenish the air in its bubble.  They occur in northern and central Europe and in parts of northern Asia. Their bite is not fatal to humans, but their fangs can pierce skin and cause fever and inflammation.

Marine animals

Marine animals have a greater reputation for danger than freshwater animals, in part because of their larger size. Very few people would welcome the sight of sharks swimming toward them. But many marine animals are more dangerous to humans than the ocean’s apex predators. All use venom as their weapon of choice.

You are far more likely, for example, to come into contact with a jellyfish than you are with a Great White Shark.  While many species cause only mild irritation, many hurt like hell and a few are deadly.

The most dangerous is the box jellyfish, of which there are around 50 species. In the Philippines alone, 20 to 40 people die from box jellyfish stings annually. In 2018, a seven-year-old girl died after being stung by a box jellyfish while holidaying with her family on Sabitang Laya Island. She was wading in the shallow water by the beach when she became entangled in the jellyfish’s tentacles. She had died by the time they reached the hospital an hour later.

Chironex fleckeri, the Australian box jellyfish, is the largest species and can grow up to three metres long. It causes the most human fatalities. The venom attacks the nervous system and skin cells and can lead to cardiovascular collapse within minutes. Some say that the pain alone is so excruciating that it leads to heart failure or drowning from shock. Many do survive a box jellyfish sting, but the pain can continue for weeks and lead to permanent scarring.

Scars from a box jellyfish.


On the other end of the size spectrum, the species of box jellyfish known as the Irukandji jellyfish measures just 5mm wide and between a few centimetres and one metre long. It was originally observed in North Australia but has now turned up as far away as the UK.

Although it reportedly has the most venomous animal toxin on earth (over 100 times deadlier than cobra venom), it is not considered one of the more dangerous jellyfish because of its miniscule size. The sting can occasionally be fatal, but many survive with correct treatment. Still, encounters with this pint-sized creature are no picnic. Non-fatal stings can cause excruciating muscle cramps, severe pain in the back and kidneys, a burning sensation on the skin and face, vomiting, headache and increased heart rate.

In January 2019, 22 people were hospitalized in southern Queensland, Australia because of stings from this little jellyfish. A local researcher noted that “in Queensland alone, we put more people into hospital from Irukandji stings than shark attacks, crocodile attacks and snake bites combined.”

Other dangerous jellyfish to avoid include the Sea Nettle, Lion’s Mane Jellyfish and Portuguese Man o’ War. (Although generally included with jellyfish, this is technically a hydrozoan, not a true jellyfish).

Irukandji Jellyfish. Photo: ABC News


Stonefish and lionfish are particularly dangerous to divers, swimmers and waders. Neither actively attack, but they use their potent venom as passive defence mechanisms. Stonefish are the more venomous of the two; in fact, they are the most venomous fish in the world. Found in coastal regions of the Indian and Pacific oceans, the venom from their dorsal barbs causes temporary paralysis, heart failure and can kill an adult in less than an hour. The homely-looking fish lies peacefully in the shallows, and most victims accidentally step on it while wading.

The pain from a stonefish sting is famously excruciating. One man colorfully compared the sensation to “holding an oxy-acetylene torch on your foot, and then working its way up your whole leg over an hour or so, then smashing your leg with a sledgehammer every 10 seconds…My eyes were rolling in the back of my head. I was clenching a towel with my teeth, going blue in the face.”

Found in the South Pacific Ocean, lionfish can deliver a venomous sting that won’t kill you but can cause extreme pain, respiratory distress and occasionally paralysis.

Red Lionfish.


Snails are not likely to spring to mind as a dangerous sea creature, but cone snails are the exception. Of the more than 800 poisonous species, just a few are dangerous to humans. A sting from some of the larger, fish-eating species can be fatal.

As with any snail, it is unlikely that it will chase you down successfully. Instead, it “harpoons” its prey with hollow teeth and uses this to inject venom. You are most likely to get stung by a cone snail if you step on it or, drawn by its attractive shell, you handle it. The most toxic species occur in the Indo-Pacific.

Symptoms include intense pain, swelling, numbness and vomiting. Severe cases involve muscle paralysis, changes in vision and respiratory failure leading to death. One species, Conus geographus, has the black humour nickname of cigarette snail, since supposedly a victim has just enough time to smoke a cigarette before dying. The venom of other species has medical uses: One yields an analgesic that is a thousand times more powerful than morphine.

Cone snail. Photo: Wikipedia