Excel♥Saga features the title character frequently being dropped into a pit of alligators, piranha, and/or various other nasties by her superior-slash-object of obsessive affection, Il Palazzo. When Il Palazzo's wrath is imminent the rope to open the trap door for the pit spontaneously descends (by mechanisms unknown) into his reach. In the manga, the animals in the pit become increasingly ludicrous (baboons!), while the main characters lampshade the pit's creatures on a regular basis, for example wondering if Lord Il Palazzo personally hunts the animals put down there, since there shouldn't be any members in the organization besides the main characters.
Rando hangs Yusuke over a pond of piranha-like fishes in YuYu Hakusho.
And it was on a rack labeled "Oceanic Repellent Bat-Sprays" and there were three other varieties for different animals around it.
In The Joker's Five-Way Revenge (a notable comics story marking the return of the psychotic murderousversion of the character), the Joker threatens an old henchman (who he suspects of having betrayed him) with this... unless Batman volunteers to take his place. Naturally, Batman accepts. Naturally, the Joker reneges and dumps them both in the pool. Naturally, Batman beats the trap, rescues the hostage, and collars the Joker.
In the comic Gorsky and Butch, the Mad Scientist Dr. Fishstein/Fishmeyer/Fisherking has this as his preferred method of doing evil. Being completely incompetent, he keeps forgetting about water....
Played with in Sillage by Ehmte-Ciss-Ronn. His race is naturally aquatic and his shark-equivalents smart enough to recognise him as their master. So instead of activating a trap door, the panic button under his desk instantly floods the whole office.
In the EC Comics story "Piecemeal" (Shock SuspenStories #8), a wealthy naturalist with a large collection of rare fish is murdered in typical EC fashion by his buxom wife who falls in love with his younger brother. The latter two then go out for their usual midnight swim, only to encounter in the pool the naturalist's latest acquisition: a shark.
One The Far Side cartoon features medieval soldiers storming into a castle on the drawbridge. One of them looks down to see what's in the moat, and shouts "Ooh! Goldfish, everyone!"
Benalish Hero:(grinning) Jones! Someone put a shark in my bath! Jones:(scared) R-r-really, o Queen? Benalish Hero: Next time make it a big one.
Films — Animation
In The Emperor's New Groove a pull on the wrong lever drops characters into a crocodile pit. Even the lever's creator questions the wisdom of this setup.
In the 1986 Transformers: The Movie, anyone the Quintessons found "innocent" was dropped into a tank filled with Sharkticons.
In Despicable Me, Vector's living room sits on top of a shark tank — with a transparent floor.
Films — Live-Action
Cleverly inverted in Deep Blue Sea: the shark tank in question was built by the protagonists and most of the movie consists of the sharks breaking into the non-tank parts of an aquatic research base.
In Austin Powers, Dr. Evil wanted a pool full of sharks (with laser beams attached to their heads), but had to settle for ill-tempered mutated seabass. In the third movie, Dr. Evil did get sharks with Frickin' Laser Beams attached to their heads. as a gift from his newly evil son Scott.
In Thunderball, Largo tosses a mook who has failed him into a swimming pool. Not so bad, except it has a Trap Door that opens to let in a school of hungry sharks. Bond winds up in it too, but (no need for spoiler warning here:) Bond lives.
Hugo Drax drops Bond into a pool with a reticulated python in it in Moonraker.
Franz Sanchez feeds Bond's friend and collaborator Felix Leiter to a shark in a marine research facility in Licence to Kill. He survives, but is badly mutilated. Bond is certainly NOT pleased when he finds out.
In Shark Night, Sheriff Sabin tries to lower Nick into one of these. Nick sets him on fire and he falls in instead. The fire gets put out, but he's devoured by a sand shark.
In The Phantom, the Sengh Brotherhood has a Shark Pool in their Elaborate Underground Base. This is one of the parts of the film lifted directly from the very first Phantom story, published way back in 1936, so the trope is at least that old.
Subverted in Ace Ventura: Pet Detective, when Ace stumbles into a literal Shark Pool that turns out to actually vindicate the man he is investigating, who was shaping up before that to be a classic arch-villain. (Ace thought it contained the stolen Miami Dolphins mascot.)
The opening scene of the French movie Le Magnifique: A spy is trapped in a phone booth, which is then lifted by an helicopter (!!) and dipped into the sea, where a squad of frogmen attach it to a shark's cage before opening the door. The scene is purposefully over-the-top as it's a parody of the whole James Bond/OSS 117 type of spy literature.
In the Stormbreaker movie, Alex Rider is dropped in a tank with a giant Portugese Man o'War. That was in the book, too.
One villain in Speed Racer keeps a tank of pihranas in his truck which he uses to threaten Taejo.
In the 1936 Flash Gordon film serial, while in the underwater city, Flash is locked in a tank to fight a shark.
In Kick-Ass 2, the main villain has a shark tank as part of his personal supervillain lair. However, the shark does nothing but lay at the bottom of the pool, causing people to think it's dead. As it turns out, it's just hungry.
Colombiana. A ponzi embezzler has a covered pool in his Big Fancy House, which he delights in telling his Paid Harem is full of sharks that will attack at a single drop of blood. Cue the hitwoman protagonist infiltrating by swimming through the pool, unmolested by the sharks as she's not bleeding. She then kills his bodyguards, lures the mark out onto the pool covers, kneecapping him so he falls through an open cover. Now that there's blood in the water, the sharks attack him in a frenzy. Cue headlines: SHARKS EAT SHARK.
Symrustar had over a thousand finned and scaled pets here. From the crowning bowl where she now scattered morsels of the secret food she mixed herself (Amaranthae had heard it said that its chief ingredients were the ground flesh, blood, and bones of unsuccessful suitors)... (after next page there is little doubt).
Lord Vetinari is said to have a Scorpion Pit. Various characters have mentioned it, but we've never actually seen it. In Guards! Guards!, Vetinari himself is thrown into a dungeon and reveals that his predecessor was a little too gung ho with this trope, and had used scorpions, snakes, and rats. Vetinari actually befriended and advised the rats, so now they were the only ones left (and quite loyal to him). Whether or not this is the same place as the aformentioned Scorpion Pit is unknown.
Paul Kidby did an illustration of the scorpion pit for The Art of Discworld. In it is a mime. Hung upside down. Opposite an (unseen) plaque saying "LEARN THE WORDS". Do not try to do mime in Ankh-Morpork.
The "slow death caused by being lowered into a pond full of killer fish" idea is Older Than Feudalism. Seneca wrote about a wealthy Roman who was really pissed off at one of his slaves for breaking a glass when the emperor was visiting. So he decided to have the slave thrown into a pool full of famished moray eels, because that way it'd take longer for him to die. Good times! Thankfully, the story ends with the slave being spared because the emperor finds the idea barbaric.
Seneca's idea is recycled by Robert Harris in the novel Pompeii, where the main villain does this to a slave who kills a tank of rare expensive fish. In front of the slave's old mother.
The second book of the Caretaker trilogy, Whirlwind has a particularly nasty version: a candiru pool. The Big Bad actually breeds the things just so he can have this nifty pool to threaten people with. People thrown in take many days to die, during which time they are in agony. Some even try to drown themselves in order to avoid this, but "the body's impulse to stay alive is remarkably strong, even with the certain knowledge that one would be far better off dead."
Matthew Reilly loves this trope. It all started when the Big Bad noticed killer whales hanging around in the dive pool, and got re-used with caimans, Komodo dragons and sharks in his next books.
In The Lies of Locke Lamora, crime boss Capa Barsavi has an enclosed pool beneath his ship-based headquarters, in which he always keeps "something nasty" for when he drops victims into it.
The city of Camorr also uses a variant as part of its justice system. At the Shifting Market, prisoners condemned for crimes such as rape and murder are granted a reprieve if they can fight off an angry devil fish (which is apparently like a really pissed off octopus) with naught but a tiny dagger. Few people, if any, succeed at this task.
In The Illusion, a scientist is dropped into a pit of Taxxons — giant, eternally-hungry, alien centipedes which eat everything they can get their mouths around.
There's a more literal narrow aversion in The Escape. Marco is halfway to shark morph in the school swimming pool and really wanting to rip into some boys who are bullying him. It's only Jake talking to him that convinces him to reverse the morph and let it go.
A particularly chilling example of this trope occurs in the autobiography When Rabbit Howls written by the multiple personalities of a woman. As a punishment her sadistic stepfather lowers her into a well infested with snakes. The trauma of this single event finally pushes her over the edge and she begins to split off her personalities to deal with her abuse.
One sketch in the BBC series The Wrong Door has an evil genius discussing this with a workman fixing his Trap Door. The workman points out that piranhas aren't that deadly, being omnivores, and pointing out that his last victim is still alive. He then recommend polar bears to be the optimum creatures for a killing pool.
The antagonist of The Outer Limits episode "The Camp" kept a giant squid-like monster in a tank and used it as a means of executing prisoners. On occasion, a tentacle would be broken off and used as food.
A Saturday Night LiveJames Bond movie parody has archvillain Christopher Walken taking Bond through his lair, still under construction due to contractors being behind schedule. Walken shows Bond architectural renderings of what he would do, including dropping him into a shark tank.
In The Vampire Diaries the moonstone is placed in a well filled with vervain, making it an Acid Pool to vampires. It also contains snakes, though possibly only by coincidence.
In the Batman episode "The Spell of Tut", King Tut uses a Crocodile Pool as a Death Trap for Robin.
It is possible to create something much worse in Dwarf Fortress — the Carp Pool!
Worse yet — a croc pool. Alligators are 15 times bigger and proportionally meaner. And when they finally bite it, there are much more valuable bones and good leather. Tamed crocs outperform turkey in churning out tons of edible eggs... while sitting in their pool and biting in half any goblin who happened to drop in. Or be chained on the ground like guard dogs, since they're amphibious. Thus, it's possible to have a goblin dodge two guard crocs only to stumble off the road into croc pit with a handful of their brothers and sisters.
Or you could just use one of the many species of shark.
Pools filled with leeches are very common in Blood 2. And 90% of the times, the game REQUIRES you to swim through it.
One mission in Hitman: Blood Money features one target who performs onstage above a shark tank. The pyros can be rigged to set her on fire, forcing her to dive into the tank, where she is promptly eaten.
Mechanical shark tanks are an uncommon hazard for motorcycle stuntman Joe Danger.
Exaggerated in Paper Mario: The Thousand-Year Door, where every single body of water, from the shoreline of Keelhaul Key to the docks of Rogueport to an innocuous, luxurious water fountain in Poshley Heights, has piranhas waiting to bite at Mario should he fall in.
In Guild Wars 2, the final fight in this Vexa's Lab mini-dungeon occurs over a pool of sharks. The boss regularly shatters portions of the floor and has a number of push/pull skills, causing hapless players to fall in.
The aforementioned acidborn shark from Dungeonscape is prominently featured in strip #541 of The Order of the Stick, where O-Chul is dropped into a vat of spikes, acid, and an acid-breathing shark. The villains make wagers on his survival in a game show format. Coincidentally (or rather, not), Rich Burlew, the author, was involved in writing said add-on. He says, "If I thought I could have slipped laser-headed sharks through the WOTC editing staff, I would have."
The fictional movie trailer Shark Pool, naturally enough, features one of these. However, it's a swimming pool, and the only thing making people go into it is abject stupidity, despite the fact the Only Sane Man keeps telling them not to.
The Joker does this in the episode "The Laughing Fish", in a scene adapted directly from the example under "Comic Books".
"Mad Love" does this with piranhas. The Joker had concocted it as one of his many potential ways of eliminating Batman, but gave up on it because there was no way to make it funny. He had wanted to call it the "Death of a Thousand Smiles", but piranhas are incapable of smiling, even when given Joker-Venom. Harley Quinn tried to implement the plan herself to impress him, reasoning that the frowns would look like smiles if you lowered Batman into the tank upside-down. Joker was furious, however, because she had to explain the joke.
In "The Underdwellers", the Sewer King tries to throw Batman into a pool of alligators, but falls in himself. However, the Sewer King laughs and explains that his alligators are 100% loyal and would never harm him.
In the episode "Unmasked", a member of Kobra who let the plan slip is tossed into a pit of snakes. He doesn't get a last-second reprieve, either.
In the episode "Out of the Past", two mooks try to throw Terry into an alligator pool, but he breaks free and throws one of them in instead. To his credit, the mook fights off the alligators with his machete and escapes.
A late-entry Wolf and Sheepdog cartoon has Ralph Wolf rigging up a can't-miss deathtrap for Sam Sheepdog, with an armory of weapons at point-blank range, the bluff he's on undercut and rigged to break off over a big tank full of hungry crocodiles — just as he's throwing the master switch, the 5:00 whistle blows. He rolls his eyes and sighs "Pshaw!" and companionably heads home with Sam.
Sam: Better luck next time, Ralph. Ralph: Oh, sure! You can't win 'em all, Sam. Nice day, huh? Sam: Yep. Good to be alive, Ralph.
In Honeys Money, Yosemite Sam tries to kill his overgrown stepson Wentworth by putting alligators in the swimming pool. Unfortunately for him, Wentworth makes such a big splash that it sends all the alligators on top of Sam.
Seņor Senior Sr. also had a shallow pool with crocodiles.
Professor Dementor had an almost generic shark pool, filled with lava!
In "Bad Boy" Evil!Ron had a piranha pool prepared in order to scare Shego into complete obedience. It worked.
In The Venture Bros. episode "Are You There, God? It's Me, Dean", the Monarch has the main characters tied up and prepared to drop them all into a river infested with candiru fish. Venture mocks him, claiming the candiru's ability to swim up a man's urethra is an urban legend, too bad he's wrong. (You may now cringe if you're a guy.) He has also used an actual pool of sharks to aid in the execution of a purportedly disloyal henchman. Although the sharks didn't touch him, because he'd already replaced the guy's blood with acid.
The Pirates of Dark Water features Bloth's Constrictus, a nasty alien creature that lives in a watery pit in his deck, and that he uses to dispose of enemies and stupid crewmen. Quoth the first episode:
Niddler: I want to be fed! Bloth: Niddler, when have I ever lied to you? I'll feed you — to the Constrictus!
Megas XLR has Kiva and Jamie being suspended over a quantum singularity. Because it's funnier.
In Teen Titans, the villain Control Freak sics a mechanical shark named Glenn on Aqualad, because he's Genre Savvy enough to know the hero could command a real one.
Chairface Chippendale: Unfortunately, the three of you aren't going to be around to witness my historic crime, because I'm going to feed you to my pit of ferocious man-eating alligators! Arthur: What?... What? (aside, to The Tick) What?! The Tick:(ahem)Standard villain procedure.
Dr. Doofenshmirtz from Phineas and Ferb has done this to Perry the Platypus a few times, such as in "The Ballad of Badbeard" with a pair of alligators.
Snap Trap from Tuff Puppy has a shark tank as his favored methiod of tormenting Larry. He also ties Kitty and Dudley up and prepares to dip them into it as a Death Trap, but presses the wrong button and gets a Lava Pit instead. He decides to go with it anyway.
One episode of The Simpsons, "Bart The Daredevil", had daredevil Lance Murdock intentionally invoke this by agreeing to jump over a tank filled with man eating great white sharks, ravenous piranha, bone-crushing alligators, and the most frightening of all, a ferocious lion (the lion is actually added into the pool with the other aquatic predators). Lance makes the jump, but then falls off the rim into the tank. Luckily, he manages to survive. Naturally, it was the lion that nearly killed him.
The Golden Nugget Casino Hotel in Las Vegas has a shark tank next to the swimming pool. It even has a transparent water slide that goes through the tank.
The Atlantis Resort and Casino in Nassau has a similarly arranged shark tank, pool and water slide.
The Georgia Aquarium has a shark pool with four foot long Bonnet Head Sharks — that visitors are encouraged to touch. (They're fairly even-tempered sharks.)
Most aquariums have tanks full of aquatic predators.