As two of the four classical elements, and probably the two most frequently used in fiction, it's no surprise that fire and water can play dominant roles in building atmosphere in visual media. A raging rainstorm can carry as much pathos as a burning house, just as a roaring bonfire can be as comforting as a calm lake. When a writer wants to form an immediately obvious contrast between two characters, places or events, though, it can be particularly visually stimulating to associate one with fire, and one with water--perhaps drawing a contrast between creation and destruction, tranquility and belligerence, or nurturing and consuming.
Alternately, it can be equally stimulating to create a contrast between fire and ice, with the two playing into differing flavors of destructive power, or mutually destructive elements that help form the world.
If writers want to play with religious motifs, this trope can play into imagery frequently associated with baptism, with a "baptism by fire" forming a contrast to traditional baptism by water. If this trope is employed, expect lots of playing around with the Orange/Blue Contrast.
Compare Evil Is Burning Hot and Evil Is Deathly Cold (when this trope is employed in two differing portrayals of evil), and Elemental Rivalry (when this trope is used to set up two elemental wielders as adversaries).
Anime and Manga
One of the openings of Yu-Gi-Oh! GX featured Bastion Misawa with Water Dragon and a similar fire dragon. However Water Dragon was the only one of the two to have ever been used in the anime (and the only one of the two to be Defictionalized).
In Dragon Ball GT we have the two brothers Nova Shenron (fire) and Eis Shenron (Ice).
In ''Fairy Tail there are Natsu (Fire) and Gray (Ice).
In Rurouni Kenshin: Kenshin and the Kyoto arc Big Bad Shishio Makoto can be considered rivals (both were highly skilled hitokiri during the Bakumatsu period (1853-1868), and now they're enemies. In their final battle, Kenshin's revamp and secret attack summons forth a fierce squall, while Makoto's special attack and power up are announced by roaring flames.
Rin Natsuki/Cure Rouge and Karen Minazuki/Cure Aqua from Yes! Pretty Cure 5. The former uses fire elemental magic and is designated as the Cure of Passion. The latter uses water elemental magic and is designated as the Cure of Intelligence. These two characters early on are depicted as rivals that couldn't get along most of the time and hate being one-upped by the other. However, over the first season, they came to an understanding, and the rivalry became less severe.
The aforementioned rivalry between Namor the Sub-Mariner and the original Human Torch also carries over into the current comics, where Namor is a frequent supporting character in books featuring the Fantastic Four (of which the second Human Torch is a member). Namor, a proud, overbearing monarch known for his stoic personality, provides the perfect foil for the compassionate, fun-loving daredevil Johnny Storm.
Jean is a red-haired woman who has gone by the code name "Phoenix" for a good bit of her career, frequently wears a flaming bird insignia as part of her costume, and has psychic powers that frequently manifest themselves as flames. Appropriately, she is known for her warm, compassionate nature, and (on darker occasions) for her unpredictable fiery temper.
Emma (while not actually having ice-based powers) is frequently visually associated with ice and the cold because of her surname "Frost", her blonde hair, her all-white clothing, and her diamond-based form that makes her resemble an ice sculpture. Appropriately, she is known for her coldly stoic personality, and her often cynical attitude towards life.
Justice League of America's aptly-named Fire and Ice, who have something of a surrogate sisterly relationship with one another, are a fairly straightforward example.
Batman and the Outsiders has a villainous couple with heat and cold powers, who are actually trying to find a cure to their opposite powers.
The first page of issue #1 of 100 Bullets is a flashback sequence showing the the central character Dizzy Cordova standing in the rain with a gun to her head, and it's immediately followed by a scene of a nude Dizzy taking a shower. The final page of the final issue has Dizzy in the burning Medici manor, pointing a gun to Agent Graves' head, possibly about to be burned to death.
Discussed in Freddy vs. Jason where it's suggested that some of the animosity between Freddy Krueger and Jason Voorhees stems from the fact that the former was created from fire, and the latter was created from water (Freddy became a dream-dwelling demon after being burned to death by an angry mob, and Jason became a hulking zombie after drowning as a child).
Employed in V for Vendetta, as a contrast between V and Evey's respective "rebirths": we see shots of Evey basking in a rainstorm following her imprisonment and torture at V's hands interposed with a flashback of V emerging from the fiery ruins of Larkhill Concentration Camp.
The climax of Casino Royale has 007 fighting to save Vesper Lynd from the bad guys amidst a collapsing building in Venice, and ultimately diving into the water in an attempt to save her from drowning.
The climax of Quantum of Solace has 007 and Camille fighting the bad guys in a burning building in a desert in Bolivia, and just narrowly avoiding being consumed by flames.
In Sherlock Holmes, the two most dramatic murders carried out by Lord Blackwood (both of which occur at the midpoint at the movie, just as Blackwood's plan is coming together) employ this trope. First he drowns Sir Thomas Rotheram in his bathtub, then we see him burn Ambassador Standish alive about 15 minutes later.
Used as a plot point in Constantine, where it's explained that water is a "universal conduit" that can facilitate transportation to Hell and its surrounding realms. In order to reach Hell, which is made of fire and brimstone, a person has to suspend at least part of their body in water while on Earth.
Played extremely straight in The Adventures of Sharkboy and Lavagirl, through the obvious differences between Sharkboy (a bitey, angry water-guy) and Lavagirl (the feisty, but caring fire-girl). Although the characters themselves seem like they wouldn't be able to get along (and indeed, they are often hurt by each other), together they make up for the other's weaknesses.
The first scene of Seventh Son, the first book in the series, has the protagonist nearly dying as an infant when his family is caught in a flood (which ends up drowning his oldest brother).
The first scene of Red Prophet, the second book, introduces us to a man with the ability to control fire, who nearly ends up killing the book's Big Bad by burning down his fortress.
Used for the Triwizard Tournament in Harry Potter and the Goblet of Fire, where the first challenge requires the competitors to face off against four fire-breathing dragons, and the second challenge requires them to swim to the bottom of the Black Lake while battling Merpeople.
There are two conflicting religions based around the ocean-dwelling Drowned God (whose followers show their devotion by anointing their heads with seawater) and the fiery "Lord of Light" R'hllor (whose followers show their devotion with huge bonfires).
This trope is used to contrast several of the warring factions in the War of the Five Kings.
One major faction is led by the dragon-taming House Targaryen, whose members follow the motto "Fire and Blood", have a dragon as their sigil, and claim to trace their lineage to an ancient civilization that was destroyed by volcanoes. One (in)famous member of the family started a lot of drama by attempting to use alchemical weapons to burn his kingdom's capital city to the ground.
One faction is led by Lord Stannis Baratheon, a militant follower of the religion of the aforementioned god R'hllor, who wears a red gold crown with points fashioned to look like flames, and has a ring of heart-shaped fire worked into his personal sigil.
One faction is led by the island-dwelling Greyjoy family, whose members have a kraken as their sigil, anoint their leaders with crowns made of driftwood, and follow the religion of the aforementioned Drowned God.
One faction is led by the Stark family and their Northern allies, who are constantly associated with ice and cold, following the motto "Winter is Coming", and are (initially) led by a man who carries a greatsword called "Ice".
Two major inciting events in the series involve the simultaneous return of the Others (undead creatures from the frozen North who carry weapons made of ice, and melt like ice when killed) and the fire-breathing Dragons. Both happen on opposite sides of the world, and both happen in the domains of two opposing factions of the war.
The first Bad Future in Heroes has a fire-using Peter battling Sylar, who is using ice powers.
Mythology and Folklore
In Norse Mythology's creation myth, runoff from Niflheim (the world of ice and mist) got too close to Muspellheim (the world of fire and heat) within the Gaping Nothing, which eventually led to the creation of the Universe.
In Hawaiian mythology, Pele the fire goddess is in a constant battle with her sister Na'maka the ocean goddess. This conflict actually creates Hawaii, continuously, as runoff from the volcanoes reaches the sea and solidifies.
Most Toa teams in BIONICLE have a Toa of Fire as The Hero and a Toa of Ice as The Lancer, typically having a Red Oni, Blue Oni dynamic. Tahu and Kopaka are the most prominent example, although they're both just as likely to start arguing with Gali, the Toa of Water, as they are with each other.
In Metal Gear Solid 3, the final two members of the Cobras are set up using this trope. First, Snake battles "The Fury", who's perpetually angry, and battles his enemies by burning them with flamethrowers and jet engines. The very next boss battle takes place in a river in the middle of a raging rainstorm, where Snake is pitted against "The Sorrow", who's eerily calm, and never even throws a punch at him.
In the Diablo series, the hell you visit in Diablo II has overall a lava and rock motif, while the heavens you visit in Diablo III has a water and crystal motif (at least, before Diablo starts corrupting them). This is especially notable at the part where you must take some portals from the heavens to hell: the contrast is quite evident there.
Killer Instinct contrasts Glacius, an ice-themed alien shape-shifter, with Cinder, a human who was mutated into a living flame by an experiment gone wrong.
In Pokémon Ruby and Sapphire, the legendary Pokémon Kyogre and Groudon are linked with the oceans and the continents respectively, but Groudon can also use fire attacks and creates a 'Sunny Day' effect when on the battlefield. There are also two groups of Pokémon trainers locked in a feud and battling for control of these legendaries: Team Aqua wants to use Kyogre to flood the continents and expand the sea, while Team Magma want to harness Groudon to evaporate the sea and raise new land.
Samurai Warriors has Kenshin (ice) and Shingen (fire).Note that it may refer to the chosen element of their ultimate weapon.
Prince of Persia The Forgotten Sands has the elemental opposition between Razia the Marid Queen (who has powers over water, which in the series symbolize life) and the Ifrit Lord Ratash, who has power over both fire and sand (which, in the series, is opposed to water as a symbol of death and entropy.)
Dark Dawn: Two gems that give you the power to evaporate water or turn it to ice are held by the Sand Prince and the Frost Queen. The former is a Worthy Opponent who battles you to gauge your strength before he willingly gives you the gem, the latter is a textbook evil queen who won't give up her gem peacefully.
In Avatar: The Last Airbender, which is practically built entirely on the Four Element Ensemble, the first protagonists introduced in the show hail from the peaceful and simple Water Tribe, while the main antagonists are from the despotic and warlike Fire Nation.
In Adventure Time Flame Princess ( fire elemental ) sees Finn crying over her. She thinks he's a water elemental because he "releases water," which is why they can't get along.
In the Star Trek: The Animated Series episode "The Magicks of Megas-tu". During Captain Kirk's magical battle with the Megan prosecutor, the prosecutor summons up a wall of flames in front of Captain Kirk. Kirk responds by creating a cascade of water to put out the fire. The fire represents the prosecutor's desire to destroy Kirk, while the water represents Kirk's basically defensive and non-aggressive posture (as appropriate for a representative of the Federation).
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.