A.k.a The Law of Inevitable Immersion.
If there's a swimming pool, and the people around it are fully clothed (i.e., not in swimsuits), someone's going to end up in that pool—either by falling in or being pushed. Sometimes by jumping out of a building.
If the program is a Police Procedural, the immersion will have already occurred, and the police will be investigating its tragic results (or possibly its antecedent, as not all bodies found dead in the water necessarily died there). If it's an action genre, someone's going to fall into that pool and die in a diffusing cloud of pink.
A form of Chekhov's Gun. Frequently enforced (at least in live action material) by the aggravations and hazards of filming near a pool — protecting the equipment from moisture damage and electrical shorts, risk of someone falling in by accident and ruining makeup/wardrobe/props, etc. — that all add up to avoiding using a set with a pool unless the script demands a pool.
- If you see a body of water, any body of water in Ranma ½, someone WILL fall into it (or have it spilled/splashed on them), triggering their Jusenkyo curse and transforming them into an animal, and/or the opposite sex as a result. That's sort of the point of the whole series. And that includes body of water, glass of water, pot of water, etc...
- Ash of the Pokémon anime falls victim to this trope whenever Bianca is around.
- In Harry Potter fic Time to Live three Death Eaters following a tracking charm on Harry Portkey to the Muggle hotel where he's staying, appearing over (and falling into) the pool. Harry later jokes that they "attacked the pool and lost."
- "Arm Candy", part of the Sorrowful and Immaculate Hearts series, opens with a party around the pool at Wayne Manor. By the end of the fourth paragraph someone has been pushed into the pool fully-clothed for being a jerk.
- In Picnic Panic a drunken Ron bumps into a four-year-old guest at the neighborhood barbecue Hermione's parents are hosting, sending her flying into the pool. Fortunately Harry fishes her out unharmed.
- In Amalgum - Lockhart's Folly Lockhart conjures a temporary floor over the pool he had installed in Hogwarts' dungeons, then vanishes it after most of his fourth-year Defense Against the Dark Arts class unsuspectingly walks on top of it.
- In Eyes Viktor tries to dump Nick into the pool at the Malfoys' French manor as a tease, but ends up in it himself after Nick's dog bites him in the rear. Nick then pushes in Adrian, who returns the favor by pulling Nick in after him.
- Sunset Boulevard memorably opens with its main character floating face down in a pool.
- Double Subverted in the Abbott and Costello film Hit the Ice. Lou is pushed backwards right to the edge of the pool, but doesn't go in. He then turns to the camera and mocks the audience for assuming he'd fall in, only to promptly do so.
- The Party stars Peter Sellers as a bumbling fool who almost falls into the extensive pools throughout the party venue every couple of scenes, until the inevitable happens - to pretty much everyone in the movie.
- In Miss Congeniality, Eric and Gracie have a heartfelt conversation in/by the hotel pool (he had been swimming laps, and she had just stormed in after getting fed up with her pageant training). Immediately afterward, Eric grabs Gracie's wrist and yanks her into the water. Gracie notes that Victor is going to kill him for ruining the gown she'd been wearing.
- Averted in Little Miss Sunshine; at one point two characters get into a poolside argument. It seems like a clear set-up for one to throw the other into the pool, but it never happens.
- Whatever It Takes, with a full ballroom of people in long dresses or smoking.
- La Notte by Michelangelo Antonioni features high-class socialites jumping into the outdoor pool during the party. Though the heavy shower that had begun just before that had left nobody dry anyway.
- Sneaks up on the viewer in It's a Wonderful Life. The high school dance seems to be in an ordinary gym, but the floor is mechanized and has a pool underneath. A prankster notices this feature and activates the floor to dump the dancing George and Mary into the drink.
- Subverted in Cannonball Run. The hotel where the race starts has a pool, but you don't see it until Terry Bradshaw drives his stock car into it.
- A major plot point in Now You See Him, Now You Don't where the bank robbers drive the invisible car coated with the water-soluble invisibility formula through a fence and into a swimming pool.
- Taken to extremes in Midnight Madness. While the yellow and blue teams race for the finish, the red, green, and white teams argue near the pool. Eventually, the red and green teams start pushing each other's members into the pool. When two members of one team are left, all four members of the white team come up to push them in. After doing so, all four members of the white team lose their balance and fall in themselves.
- In Gordy, a young boy is sitting on the diving board over a pool as he talks to his new friend. Of course, when he gets up to leave, he loses his balance and falls in.
- The "Dancing Queen" number in the film version of Mamma Mia! has women jumping off a pier for no reason.
- In My Favorite Wife (1940), Ellen tries to make a grand exit when Nick and Stephen start arguing over who has more right to be with her, but she doesn't look where she's going and falls into the swimming pool.
- Agent Cody Banks has Cody being thrown into the pool at Hilary Duff's casino-themed party by a bunch of Jerk Jocks.
- An almost example in Star Trek: Generations but instead of a pool, it's a holographic ocean. Otherwise, it fits the description of this trope.
- Almost Famous: Tripping rock star Russell stands on the roof of a house over a swimming pool surrounded by drunk, cheering fans, ranting incoherently and preparing to jump in. Beleaguered trip-sitter William attempts to talk him down. Russell says "okay" and turns to go back inside. Yeah, right.
- In Rocky III, a fully clothed Paulie is pulled into the pool by Rocky during the Training Montage.
- In Sammy the Way-Out Seal (1962), the titular seal pushes a poolside shindig guest in with his nose. Since no one saw the seal, an escalating series of retaliatory pool-pushes ensues.
- In Last Vegas, first, Billy pushes Paddy into an indoor pool during the party. Then, Paddy returns the favor the next day at the hotel pool.
- In The Satan Bug, Dr. Ostrer is found floating dead in his own swimming pool. The image was used prominently in some posters for the film as well.
- In the first Lethal Weapon, as the heroes are escorting a suspect past a covered pool, the suspect goes for a weapon and Riggs reflexively shoots him. He was trying to be nonlethal (this time) but the injured suspect falls in the pool, is tangled in the plastic covering, and drowns.
- In Gentlemen Prefer Blondes at the end of "Ain't There Anyone Here For Love?" Dorothy gets knocked into a swimming pool by a dancer leaping over her. This was actually an accident that the director decided to keep in the final cut because it was funnier than just having Jane Russell stand on the edge of the pool.
- Several sissies in the cowboy/sissy brawl at the end of Blazing Saddles wind up in a shallow pool. They start doing water ballet.
- In the first Jack Reacher book Killing Floor, in the final act Reacher sets a trap for a group of killers with himself as the bait at Hubble's house, which has a pool in the backyard. Though he manages to kill four of the group silently with his knife, Kliner's son spots him trying to sneak up on him in the backyard. After managing to avoid getting blasted with his shotgun, Reacher tackles him into the pool and drowns him.
- In the Animorphs series, David's house has a pool, and the other characters mention it a few times in the context of how they envy David's good fortune in this respect. Eventually, this comes into effect when the characters are chased by several Mooks through David's yard, and they are able to make their escape because the mooks fall into the pool.
- In the book Elantris, the characters discover a pool in a well-hidden room. It turns out to be the 'merciful' way of killing Elantrians, originally when they chose to die, later when they were overwhelmed by the pain.
- Subverted in the Hercule Poirot mystery The Hollow: the victim dies at the edge of the pool, and a bystander with... complicated motives throws the apparent murder weapon into the pool, just to make it that bit harder for the detectives.
- Someone is thrown into the Wakefield's backyard pool in pretty much every Sweet Valley High book.
- In Instrument of God, 246 pushes a woman who can't swim but is afraid she'd drown into the pool order for her to understand that because they're already dead they cannot drown.
- Brothers and Sisters:
- There was a jump-in by several people.
- Invoked right from the first- William Walker falls in during his fatal heart attack.
- America's Next Top Model: Happened on Cycle 8, when one of the models annoyed 50 Cent.
- The West Wing. During a flashback to when Toby offered CJ a job on the Bartlett campaign. CJ wasn't wearing either her contact lenses or her glasses and was carrying a box of her possessions on the way back home from the PR firm from which she had just been sacked.
- An episode of Criminal Minds had a bikini-clad starlet pull Spencer Reid — shirt, tie, shoes, sidearm and all — into a pool. Of course, with Reid being Reid, he ends up deflating the moment by telling the starlet that her agent has been murdered.
- Though not a pool, in an episode of NCIS Ziva falls into a creek, dragging Ducky with her - naturally she has just told him she doesn't need assistance crossing said creek.
- McGee and Ziva are also pushed into a fountain in one episode.
- NUMB3RS If there is a pool (or any water, really) Colby will end up in it.
- An episode of Psych features McNabb's mailbox rigged with a bomb, which explodes, sending him flying backwards — into the outdoor pool of the apartment complex he lives in.
- In Dexter the title character pushes his adopted daughter Astor into the pool at a party in an attempt a playfulness. While she is actually wearing a bathing suit, she doesn't much appreciate it.
- Averted in the third Sherlock episode, "The Great Game." Sherlock agrees to meet Moriarty at a swimming pool, but no one falls in at all.
- Kamen Rider has a tradition of heroes getting beaten up and tossed in the drink by an exceptionally strong opponent who'll probably have more characterization than a standard Monster of the Week. An early round with a general, an upgraded version of a monster the hero thought completely blownuptified, or another Rider under a misunderstanding, will happen on a bridge over a river and the Rider will be taking a bath. Kamen Rider 555 was especially fond of it.
- On the ill-fated Brady Bunch Variety Hour, the show's stage literally had a built in pool. While it was primarily used by a synchronized swimming troupe (The Kroftettes) during the credits, you can be certain that a fully-dressed Brady cast member would occasionally take a dip.
- Averted in the Mad Men episode "The Jet Set," when Don goes with some strangers to their vacation house and passes out from the heat right next to the pool, but played straight in "A Tale of Two Cities" when he comes out of a drug-induced fantasy at a party to find he faceplanted into the pool and nearly drowned, which is in keeping with the "death wish" imagery established in the season opener.
- Babylon 5: Not actually seen onscreen, but a past incident between Ivanova and a member of the Psi Corps involved a swimming pool:
Ivanova: You know how I feel about telepaths.
Sheridan: Do I ever. You threw one out a third-story window on Io.
Ivanova: There was an ample pool below the window!
Sheridan: I'll assume you knew that.
- Terminator: The Sarah Connor Chronicles takes advantage of this trope to inexpensively but effectively film a Curb-Stomp Battle against a terminator from under the surface of a pool, with an entire SWAT team plummeting into the water one (SPLASH!) by (SPLASH!) one (SPLASH!) to muffled yells and gunfire. (And also to the dulcet voice of Johnny Cash.)
- The opening of the CSI episode "Suckers" looked like this trope, until the team fished out the "body" and discovered it was a mannequin.
- One episode of Law & Order: Special Victims Unit included a man shoving his wheelchair-bound wife into their indoor pool, proving she is not really wheelchair-bound.
- One of the Rock Band 3 band progress cut scenes has your band at a rooftop party with a pool front and center. Sure enough, one of the first steps towards rowdiness is the singer pushing some random schmuck into the pool.
- In The Love Boat one man throws another into the pool after the victim says that he knew the thrower wouldn't be able to get Angela's number. Cleaning up the resulting puddles is Isaac's task for the level.
- In the 2009 Halloween episode, The Nostalgia Critic (intentionally) dives into a pool fully clothed. After he climbs back out, he gets knocked back in again, almost immediately.
- At the big Halloween party in the Whateley Universe, there's an Olympic sized pool, but it is under a concrete floor and not accessible from the big party area. Jimmy T still falls into it.
- Archer combines this with Sheet of Glass when he comes across a pool while pursuing a paranoid Kenny Loggins. Not just a pool, but a glass walled pool on the top floor of a high rise hotel. Not only is the dunking inevitable, that glass must shatter the same way the gun must fire.
Archer: The predictable thing is going to happen now, so...*crash!*
- Was central to an ad campaign in The '70s: Take the Nestea Plunge!
- Comedian Steve Byrne laments the death of this trope in Real Life in recent years. Years back, when you do this, the person mucks around the water and you all get a good laugh. Nowadays, you do this, the person screams out "Oh, my iPod! My cellphone!" and you're out a few hundred dollars. The trope may make a comeback in the future once waterproof phones become widespread.
- Casa Bonita (restaurant near Denver, depicted in South Park) has little shows (gunfight between Black Bart and the sheriff, for example) taking place over a large pool of water. You pretty much know that, during any given show, somebody is going in.
- A tragic variation occurred where a bridesmaid pushed a bride into the pool. The bride landed wrong and has been paralyzed from the neck down ever since .
- Basically, while it may look fun in the media, please Don't Try This at Home.
- German footballer Lukas Podolski seems to love this rule; he tossed a reporter into a hotel pool during a training camp in Italy before the 2014 World Cup... then did the same thing to a teammate a few weeks later.