Rule of Pool

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 getting pushed into that pool. 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.

Not at all related to the Rule of Cool.

Anime and Manga

 * If you see a body of water, any body of water in Ranma 1/2, 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.
 * Body of water, glass of water, pot of water, etc...

Film

 *  ~A.I.: Artificial Intelligence~ , and the predictability only enhances the impact.
 * 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.
 * Miss Congeniality.
 * Averted in Little Miss Sunshine; at one point two characters get into a poolside argument. It's 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.
 * The dance scene from Its a Wonderful Life.
 * In the 1998 version of The Parent Trap.
 * Subverted in Cannonball Run. The hotel where the race starts has a pool, but we 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.
 * Both pool-side scenes from the 2010 movie You Again follow this rule.
 * 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.
 * Agent Cody Banks has Cody being thrown into the pool at Hilary Duff's casino-themed party by a bunch of Jerk Jock s
 * 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 hilariously) 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.

Literature

 * 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, spots him trying to sneak up on him in the backyard. After managing to avoid getting blasted with his shotgun, Reacher
 * 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, the Rule of Pool 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.

Live Action TV

 * Melrose Place
 * Dallas
 * Dynasty
 * ~Brothers & Sisters~, although that was more a jump-in by several people.
 * It was actually 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 first season finale of The Sarah Connor Chronicles.
 * The West Wing. We see 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.
 * 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.
 * Subverted and teased at in the episode 'Blackout' of Cold Case.
 * Numb3rs If there is a pool (or any water, really) Colby will end up in it.
 * An episode of Psych features.
 * 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 pilot episode of Kyle XY.
 * Averted in the third Sherlock episode, "The Great Game." Sherlock agrees to meet at a swimming pool, but no one falls in at all.
 * In the first episode of Party Down.
 * 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.

Video Games

 * 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.

Web Comics

 * Narbonic, though the people who fell in were

Web Original

 * 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.

Western Animation

 * Naturally, The Simpsons.

Real Life

 * Was central to an ad campaign in The Seventies: 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.
 * 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.

Fanfiction

 * In Loralee's Harry Potter fic Time to Live, three Death Eaters who were following a tracking charm on Harry tried to Portkey to the Muggle hotel where he was staying and appeared over (and fell into) the pool. Harry later joked that they "attacked the pool and lost."