Taking a dip the sea can be a hazardous business today. But there were times in the past when going anywhere near the ocean would have been downright foolhardy.
Marine reptiles big enough to crush boats between their jaws and vicious, predatory whales are amongst the bizarre and deadly creatures that roamed the Earth's ancient oceans. Here, in order of increasing lethality, are the seven most dangerous seas of all time.
Number 7: The Ordovician
During Ordovician times, Earth was an alien, barren place. There were no animals living on land, and few plants. A day lasted 21 hours instead of 24 and oxygen levels in the air were just 15%, compared with 20% today. But the sea was teeming with life forms.
Sea scorpions were amongst the most common animals. These hideous primeval creatures sifted the sandy beaches with their claws, looking for tasty morsels. Anything that got too close was likely to fall victim to their nasty snapping claws.
But the Ordovician's top predators were the Giant Orthocones. These squid-like behemoths could glide through water, sweeping up a sea scorpion or a hapless trilobite with their tentacles and crushing them with a powerful beak.
Number 6: The Triassic
The Triassic seas were populated by sea-going reptiles including the lizard-like Nothosaur and bizarre Tanystropheus, an animal with the longest neck possible by the laws of physics.
But the daddy of all predators in this sea was Cymbospondylus, a sleek, powerful predator and at up to 10 metres in length, the largest ichthyosaur ever to have evolved.
On land, the first dinosaurs had evolved. But these early dinos were still tiny compared to their descendents in the Jurassic.
Stethacanthus had a fin shaped like an anvil.
Number 5: The Devonian
Devonian waters were home to a vicious range of sharks, including Stethacanthus, whose weird dorsal fin was shaped like an anvil. But these were nuisances in comparison with Dunkleosteus, an ugly, armour-plated leviathan with a lethal set of slicing tooth-plates.
Dunkleosteus would devour anything in its path - fish, sharks, cephalopods and other Dunkleosteus. It had a habit of gulping its food down and then vomiting it up afterwards.
By now, the land was covered by plants and early trees such as Archaeopteris. The first animals were now colonizing this virgin territory.
Number 4: The Eocene
By the Eocene, mammals dominated the land and were branching out into the sea. Basilosaurus was one of the earliest whales. But unlike modern whales, it was free from blubber and sported a set of vicious teeth.
These primitive cetaceans could reach around 21 metres in length and were originally mistaken by fossil hunters for sea serpents.
Megalodon was a relative of the Great White shark.
Number 3: The Pliocene
About four million years ago, the coast of Peru was a barren desert. Seals basked on the rocks and a bizarre amphibious sloth waded in coastal waters. But further out to sea, was one of the most fearsome fish ever.
A close relative of the Great White shark, Megalodon dwarfed its modern relative at 16 metres in length. Megalodon's staple diet was Odobenocetops, an extinct whale that sifted the mud on the seafloor for food. If Great Whites are anything to go by, Megalodon may have been an ambush predator, sneaking up on Odobenocetops from beneath.
Number 2: The Jurassic
Jurassic seas were home to voracious sharks such as Hybodus and the nasty crocodilian Metriorhynchus but they would not have stood a chance against the fearsome Liopleurodon: the largest predator of all time.
This enormous carnivore belonged to a family of marine reptiles called pliosaurs, and we know from its nostrils that it had an extremely good sense of smell. This undoubtedly made it an extremely efficient predator.
But while a dip in a Jurassic sea should cast fear into the hearts of even the bravest soul, the succeeding period in Earth history would see the appearance of even more terrifying sea creatures.
Mosasaurs often plucked birds out of the air.
Number 1: The Cretaceous
The Cretaceous ocean ranks as the most dangerous sea of all time due to the sheer number and ferocity of its marine predators.
You just have to look at Hesperornis. This bird spent much of its time on rocky ledges above the water. But it was frequently picked off by small mosasaurs like Halisaurus, who waited in the shallow caves beneath these ledges for a Hesperornis to dive in. When it did, Halisaurus would grab the bird in its short, sharp teeth.
But these mosasaurs were dwarfed by their huge relatives, the Giant Mosasaurs. These gargantuan fiends reached up to 17 metres in length and could take on pretty much everything in the sea.
Also common in the Cretaceous was the largest of the long-necked plesiosaurs: Elasmosaurus. This 15-metre-long plesiosaur used the length of its neck to sneak up on unsuspecting shoals of fish.
You weren't safe on land either. This period saw the evolution of some of the fiercest land predators of all time, including the notorious Tyrannosaurus rex.