There are few things more pleasant in life than a refreshing dip in the ocean on a hot summer’s day. But lurking beneath the waves are millions of not-so-friendly sea-dwellers – and they exist in every continent, from Oceania to Asia and Central America. Somehow, we’ve managed to rank the top 30, in order of their deadliness to humans.

Click through this gallery to discover the world's most dangerous sea creatures (some more surprising than others)...   

Luminous green, moray eels are hard to miss – lucky for the scuba and deep-sea divers who live in fear of encountering them. While they’re not aggressive by nature and only attack when disturbed, their incredibly sharp teeth can inflict serious wounds or injuries. Plus, while morays are not venomous, their flesh can be toxic and even fatal if consumed. 

Gentle and graceful by nature, stingray attacks on humans are thankfully rare. When threatened or provoked, they may whip the venomous barbs on their tails, but this is usually a last resort – swimming away is their preferred method of defence. Tragically, beloved conservationist and Australian TV presenter Steve Irwin was killed in 2006 when a stingray’s barb pierced his chest, causing a fatal injury to his heart. Spot one in the wild? Gaze in awe, but keep your distance.

One of the largest fish swimming the seas, great barracudas can reach speeds of up to 35 miles per hour (56km/h), weigh up to 45kg (100lbs), have two sets of razor-sharp teeth, and can achieve a length of up to five feet (1.5m). No surprise, then, that they have a deadly reputation. But is it fair? While it’s true they pose a danger to people due to their tendency to attack shiny objects (like fishermen’s spears) attacks are quite rare, with two fatalities recorded. 

Despite its cute name, the striped-pyjama squid is actually both venomous and poisonous. Native to Australia, it’s just one inch (3cm) long, looks a little like a dumpling, and appears sweet – until you learn it emits a poisonous slime thought to be extremely dangerous to humans, and backs that up with a venomous bite.​ The striped-pyjama squid is actually a member of the cuttlefish family, and is a master of disguise, able to hide its body under the seabed.

The largest species of flying squid, Humboldts live for just a year on average, they can reach a length of over seven feet (2m) – that's including tentacles – and lay as many as 20 million eggs during that time. Nicknamed ‘red devils’ by fishermen in their natural habitat, from Chile to Alaska, they are known to be aggressive when approached, and stories of Humboldts squirting ink and attacking scuba divers abound.

Often weighing up to 400kg (880lbs) and stretching up to eight feet (2.4m) long, the clue’s in the name – giant groupers really are enormous fish (the biggest bony fish found in coral reefs, in fact). Found on reefs from the Red Sea to Hawaii, they’re not typically aggressive towards humans. However, divers are strongly advised to steer clear, due to their sheer size and extremely territorial temperament. 

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Also known as carpenter sharks, sawfish are actually part of the ray family. One of the most unique-looking sea creatures on this list, they have a distinctive sawlike snout and are among the biggest fish on the planet, reaching up to 23 feet (7m) long. With a skeleton made of cartilage, they’re not ultra-aggressive towards humans, but their spiky snout can cause serious damage. Sadly, they are now critically endangered due to overfishing and marine pollution.

Fire corals belong to the hydrozoan group, meaning they are actually more closely related to jellyfish and other stinging cnidarians. They may look harmless, but can cause seriously painful skin reactions, particularly among deep-sea divers. Found in tropical and subtropical waters all over the world, when touched they emit a stinging venom that causes a burning rash and even soft tissue infections. While they’re not fatal to humans, divers should wear full wetsuits for maximum protection. 

Extremely territorial, the titan triggerfish is notorious among scuba divers and snorkellers, many of whom have accidentally disturbed their nests and suffered nips, bites and bruises as a result. While they’re not venomous, they do have incredibly sharp teeth and spiky dorsal spines for protection against predators. Divers are advised to avoid their territory, especially during nesting season, as the fish may even chase them up to the surface.

Like their land-dwelling counterpart, the strike of a scorpionfish packs a serious punch. With their prominent spiny heads and venomous fins, they come in a rainbow of shades, including bright red, and can inflict deep puncture wounds on anyone unlucky enough to step on one of their poisonous spines. While their venom isn’t fatal to humans, symptoms can include intense pain and swelling, often requiring hospital treatment.

One of the largest starfish in the world, crown-of-thorns starfish are a huge problem for coral conservation. It’s their main source of food, and one single COTS can eat its way through 100 square feet (9sqm) of coral in just one year. For humans, they can be equally problematic – these beautiful creatures (they’re often vibrant pink or purple) are covered in thorny spikes containing venomous toxins, which can cause agonising pain, tissue and liver damage. 

While its name might conjure up images of romantic nights on the beach, the Stargazer fish is anything but sweet. Capable of producing venom and electric shocks, it might be a contender for the ugliest critter on our list, spending most of its time buried beneath the sand, with only its face visible. When threatened, they can emit up to 50 volts of electricity via the eyes, Marvel-superhero style, while their backs are equipped with a venomous spine.

Electric eels aren’t actually eels at all, but fish – closer to a carp or catfish. Native to the murky streams and ponds of South America, they usually emit a low-level charge of less than 10 volts, but can ramp this up to a shocking 860 volts for defense or hunting. While electric eel attacks are mercifully rare, those unlucky enough to receive multiple jolts have suffered respiratory and heart failure or even drowned due to the shock. They’re also enormous, measuring up to eight feet (2.4m) in length and tipping the scale at 20kg (44lbs). 

Also known as the ‘banded sea krait’ due to its distinctive black stripes, the yellow-lipped sea snake is found throughout the eastern Indian and western Pacific oceans. While they rarely attack, their venom contains powerful neurotoxins, lethal to humans. Luckily for us, they’re a relatively calm species and are happy to coexist on both land and sea, searching for their favourite delicacy – fresh eel. 

Iconic animals on the verge of extinction

Some seals have a cuddly image – but not leopard seals. Armed with a mouthful of razor-sharp teeth and an aggressive nature to match, they have a history of attacks against humans who get too close, particularly in Antarctica. Leopard seals grow to over 11 feet (3.5m) in length and can weigh up to half a tonne, so their incredible strength makes it difficult to fight back. In 2003, marine biologist Kirsty Brown tragically died after being forcibly taken underwater by a leopard seal.

With its signature red, white and brown stripes, the Lionfish is (almost) as dangerous as the mammal with which it shares a name. Originally from the Indo-Pacific, it has now become an invasive species in the Caribbean and American waters, where it is an aggressive predator, threatening native marine life. Its sharp venomous spines can cause intense pain, breathing difficulties and even temporary paralysis in humans. Due to a lack of natural predators in these new environments, lionfish populations have exploded, creating serious ecological problems​.

Despite its name, the Portuguese Man O' War is not confined to one region and has been spotted as far afield as Cornwall, UK, and the Caribbean. Contrary to popular belief, it’s not a jellyfish but a siphonophore, a colony of individual organisms working together. Its long tentacles, which can stretch up to 100 feet (30m), deliver an excruciatingly painful sting via venomous cells, causing red welts and discomfort that can last for weeks.

If you’ve seen that episode of The Simpsons, you’ll know pufferfish – better known as fugu in Japan – are famous for their spikes and potentially deadly toxin. Although highly prized as a delicacy, pufferfish contain tetrodotoxin, a neurotoxin which can cause paralysis, respiratory failure and death. Due to this risk, chefs must undergo years of training and obtain a government license to legally prepare the fish. If mishandled, diners could be exposed to the toxin, which is 1,200 times more toxic than cyanide, with no known antidote.

They might sound like something you’d like to grow in your garden, but flower urchins’ scientific name is Toxopneustes pileolus – from the Greek meaning ‘poison breath’ – which should give you a clue as to their nature. While their pink-tinged hue may look pretty, the sting it can deliver is anything but, causing severe pain, paralysis and drowning for deep-sea divers and swimmers. Their venomous claws can even pierce through a wetsuit. 

Bull sharks might not get the same Hollywood attention as great whites, but they’re just as formidable. In fact, with over 100 unprovoked attacks attributed to them (27 of which have been fatal), they’re a force to be reckoned with. What makes them especially dangerous is their versatility – bull sharks can thrive in both saltwater and freshwater, meaning they show up in places where you might least expect a shark. Bull sharks are listed as 'Near Threatened' on the IUCN Red List due to habitat loss and overfishing, reminding us that even the ocean’s top predators need protection.

Named ‘Most Venomous Marine Sea Snake’ in The Guinness World Records, Dubois’ sea snake is native to the Pacific Ocean and also one of the top three most venomous of all snakes on Earth. Though usually docile – it will only bite if provoked – its potent neurotoxic venom can cause severe symptoms such as paralysis, drooping eyelids and muscle necrosis.

The tiger shark, one of the ocean's most feared predators, is known for its distinctive stripes and voracious appetite. These fearsome beasts can grow up to 16 feet (5m) in length and weigh more than 900kg (2,000lbs). Found in tropical and subtropical waters, they are responsible for the second-highest number of attacks on humans, after great white sharks. Their powerful jaws and serrated teeth can cause significant damage, although attacks remain relatively rare. 

Around the size of a thumbnail, the Irukandji Jellyfish is one of the tiniest creatures on this list. A subtype of box jellyfish (keep scrolling to meet them) it even has its own disease – ‘Irukandji syndrome’. Victims feel barely anything when initially stung – the excruciating pain, sense of impending doom and even brain haemorrhages come shortly after. Australia reports between 50-100 stings annually, mostly in northern waters near Queensland and Western Australia. While Irukandji syndrome can be treated, it is still regarded as one of the most painful experiences on Earth.

There are over 60 (yes, 60) species of venomous sea snake lurking in the ocean’s depths, but most are relatively docile. The one you definitely don’t want to encounter while taking a dip? The beaked sea snake. Its incredibly potent venom – delivered via powerful fangs – is four to six times more toxic than that of a cobra, and can cause severe paralysis, including of the diaphragm. Found throughout Asia and Australasia, they’re partial to a slither on sand too. Most bites occur when they get tangled up in fishermen’s nets. 

They may seem surprisingly low on the list, but great white attacks are quite rare – there have been 59 confirmed fatalities from these shark attacks globally since record-keeping began in 1580. Still, for the unfortunate surfers and bathers who wind up in their path these iconic sharks can be deadly. The great white is responsible for the highest number of fatal shark attacks, with 351 unprovoked bites recorded in total. In 2023 alone, three fatal attacks by great whites were reported, two of which occurred in Australia and one in California​. 

The geography cone snail is the deadliest species within the cone snail family, known for having the most potent venom and capable of killing humans with just a single sting. The venom – for which there is no known antidote – contains a mixture of conotoxins, which can cause paralysis, respiratory failure and death. Although fatalities are rare due to limited encounters with humans, the geography cone snail has been responsible for at least 30 recorded deaths. Its nickname, the 'cigarette snail' comes from the grim idea that a victim would only have enough time to smoke a cigarette before succumbing to the venom – a metaphor highlighting the rapid action of the toxin.

File the blue-ringed octopus in the beautiful-but-extremely-deadly category (see also: flower urchins). While its electric blue markings may make a striking photo, they’re actually a warning to predators – they’re only visible when these cephalopods feel threatened, by which time it’s often too late. Found in Southeast Asia and the Australian coastline, their painless bite causes respiratory failure and death, all within minutes – and there’s no antidote. The worst part? Victims typically remain conscious but paralysed as the toxin takes effect. 

Stonefish – named for their uncanny resemblance to rocks – won’t win any ocean-based beauty contests anytime soon. However, they do deserve the title of the world's most venomous fish. Masters of camouflage and often invisible to the naked eye, they strike when stepped on, shooting their venom into victims via their defensive dorsal spines. The result? Excruciating pain, paralysis, necrosis and, if untreated, heart failure – all in less than an hour. 

Weighing up to 1,000kg (2,200lbs), saltwater crocodiles are the world's largest reptiles, with males often reaching more than 20 feet (6m) in length. With fearsome faces and a reputation to match, they have the biggest ‘bite force’ of any land animal, inhabit both land and water, and can live up to 65 years. They even sleep with one eye open (all the better to size up potential meals). Saltwater crocodiles are responsible for hundreds of attacks worldwide, with fatality rates varying by region. In Indonesia alone, there were 170 attacks in the last decade, of which 85 were fatal.​

 And the award for the deadliest sea creature goes to the Australian box jellyfish. Its 10-foot-long (3m) tentacles deliver venom that attacks the heart, nervous system and skin cells, leading to paralysis, cardiac arrest and death in as little as two to five minutes if untreated. As if that wasn't terrifying enough, these creatures are also equipped with 24 eyes that help them navigate through the water. While 79 deaths have been recorded in Australia since the late 1800s, the global death toll is likely higher due to underreporting in regions like Southeast Asia. 

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