This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / River Monsters
Probably the most notorious thing on the show is the Goonch, aka the Giant Devil Catfish. It's a massive, predatory catfish, well over six feet long, that lives in the cold, murky waters of the Kali River in India. The Goonch has sharp, almost shark-like teeth◊ (unlike many other catfish) and is a free-swimming predator rather than a bottom-feeding scavenger. There are reports that it will attack and devour humans, but what really takes the cake is how it may have come to view humans as food. The villagers on the Kali cremate their dead, and then push the burned bodies into the river. These fish have potentially been feeding on burned corpses for centuries, and that just may have given them a taste for live humans as well...
In fact, this is how Jeremy lured in his monster: by staging a fake funeral.
Also, the one Jeremy caught is nearly six foot long and weigh 170 pounds, and it's still not big enough for a man-eater, meaning that the ones that hunt swimmers are still out there.
Many of the stories told about the fish, which give them their fearsome reputations, are these. The giant catfish in the Amazon that was caught with a fisherman's corpse jammed in its throat. The goliath tigerfish attacking and killing a little girl. The wells catfish swallowing ducks whole, and medieval accounts of them having been found with children in their stomachs...Trust us, it goes on. Fortunately, many of these are exaggerations or just plain stories (and Jeremy is quick to figure it out). But not all.
Blood sucking primordial fish invading freshwater rivers and lakes, dangerous to even the most powerful fish, and not to mention humans? As a New Englander, I find Sea Lampreys to be terrifying.
What Jaws did for the ocean, he's doing for every river and lake he can find. Seriously, you know a show has a pretty high dose of Nightmare Fuel when piranhas are among the least dangerous creatures featured.
Extra Jaws shoutout goes to the Bull Shark episode. Already not too much smaller than Great Whites, and meaner to boot, they're able to operate just as well in fresh water, the only shark in the world to do so. They've been found in sea-connected rivers up to 2200 miles inland.
And it's the big ones that tend to visit rivers; the larger ones can handle the stress better. In fact, the biggest bull shark ever caught anywhere was caught in a river.
Oh, and a bull shark was the actual species of the shark whose series of attacks were the inspiration for Jaws.
The New Zealand longfin eel, star of the season 3 episode "Flesh Tearers". While the 8ft long or bigger eels might have all been killed off, they are aggressive enough that they can and will pack-swarm even humans and rip them apart.
"The Mutilators" features the pacu, a vegetarian relative of the piranha that's been introduced to Papua New Guinea. It turns out that, far away from the Amazon, this fish with its crushing jaws and disturbingly human-like teeth has turned omnivorous, with a particular penchant for biting the testicles off of any man it can meet in the water. What's worse is that, as introduced species tend to do, it's multiplying beyond the control of the native species and is driving them to extinction.
The Tigerfish. The Goliath Tigerfish from the first season is big enough that it can bite a small child almost in half, aided by its demonic sharp teeth and a swimming speed of 45 miles per hour. But even the smaller species that lives in South Africa are deadly. They're pack hunters, like piranha... but tigerfish can be up to 3 feet long and weigh in at dozens of times the weight of the inches-long piranha. And they sometimes gather in swarms of thousands.
The Vundu is an actively predatory catfish species from the Zambezi river. Giants capable of eating humans are rare, but a healthy-sized adult is more than big enough to gulp down a human toddler, something they have a well-earned reputation for doing...
There also have been cases where vundu have drowned fisherman by pulling them under.
The vundu is feared enough a malevolent river god capable of obliterating dams with catastrophic floods has been inspired by it.
An episode centered on Japan speculates that the kappa may actually be a mythology-wrapped representation of the Giant Japanese Salamander, which is nightmare fuel on two levels. Firstly, its method of feeding is to inhale prey straight into its stomach to digest them alive. Secondly, it gets big enough that it can eat human babies. There's no record of that actually happening, but still. . .
The catfishes of the Congo River. More accurately, the kill or be killed lifestyle there shared together by fish and people.
Catfish in general. Forget about the old myth of catfish being scavengers: this shows proves that they are active, fast-moving, sometimes human-eating apex predators, some bearing an uncanny similarity to sharks in looks and mode of attack.
There is a horrific description of a caiman attack that left a man completely disemboweled (he barely survived and carried the scars to this day).
In the same episode, the focus of the investigation was on what killed and ate the passengers that fell victim to the Amazon's largest ever boat disaster. Despite the fact the Sobral Santos went down right in front of the docks, almost nobody made it to shore. After discovering that the fish processing plant dumped waste there, the fact large catfish and huge numbers of flesh-eating candiru have been recorded in the area, and the fact the less likely suspects (caiman, bull shark) could have learned to associate the waste with food, Jeremy came to a horrific conclusion; the boat upturned right where the fish waste was being dumped, leaving the wounded passengers in the middle of a perpetual feeding frenzy involving all of the Amazon's predators that will kill and eat people, especially scores of giant catfish. It's the equivalent of being tossed in with several hundred lions at feeding time.
A survivor's account describes screaming voices suddenly vanishing underwater as something came up to pull them into the depths.
While investigating, Jeremy found that the ship was refurbished and renamed, and he unknowingly traveled on it for over 2000 miles during his first time to the Amazon.
In the second episode of season 8, Jeremy investigates the mysterious deaths in the "Blue Holes" of the Caribbean, which are attributed by local legend to the Lusca. Now, this is a horrific sea monster in and of itself; a real-life Sharktopus big enough to swallow a person in one bite. But it's the conclusion Jeremy comes to that's really chilling: based on all the evidence, he suggests that the tunnels under the islands that create the Blue Holes are in fact being haunted by a species of undiscovered giant octopus, a creature that would be more than capable of eating humans.
Another cephalopodian predator is the Humboldt Squid; a pack-hunter that is not only perfectly willing to attack humans, but is so voracious that the style of catching them amounts to "use a horrifically barbed jig to snare one, then try to haul in the three or four of its buddies that will be eating it alive in a cannibalistic feeding frenzy once they sense it's wounded". Maybe the scariest part is when the Peruvian fisherman who regularly catch them note that anyone who falls over when Humboldt Squid are around pretty much is done for; if they aren't torn apart in a flurry of snapping beaks and barbed tentacles, then the squids will work together to drag them into the deep for consumption. Especially when you recall that Jeremy just pointed out that the low, sloped sides of the ships specialized in catching these squid make it very easy to fall overboard...