Artificial Gill

A device that enables a character to swim underwater without worrying about the Oxygen Meter, unless the device runs on air tanks. These are used to find and explore Underwater Ruins and the Underwater City.

There's also devices that allow beings who breathe differently to go to places that don't have the same atmosphere from where they frequent.

Comic Books

Batman carries a rebreather in his utlity belt.

Film - Live Action

Thunderball
Star Wars has Rebreathers, used for breathing in hazardous environments, including underwater. A notable example is from The Phantom Menace, where Qui-Gon and Obi-Wan use them to swim down to the Gungan city.
- On Admiral Ackbar's planet Mon Calamari (seen in the second Jedi Academy Trilogy book), they have flexible, jelly-like breathers you slap over your nose and mouth and it filters the water into oxygen.
  - The Yuuzan-Vong have a similar one that covers the nose and mouth, and extends down the windpipe to interface with the lungs directly. It's incredibly useful, but you might want to find a different solution; it hurts like a motherfucker because of the Yuuzan-Vong's cultural obsession with pain.
The Abyss had a gooey gel that was hyperoxygenated for super deep sea diving.
- Yeah, that's a thing. The scene with the rat was a real rat really being held under the surface of the liquid and freaking out because it had no idea what was going on. Animal welfare groups were annoyed about it because they thought it was being put in unnecessary distress.
At the end of Sherlock Holmes a Game of Shadows, Watson receives Mycroft's oxygen inhaler in a parcel sent by Holmes, who stole the inhaler from his brother and used it to survive when he pulled Moriarty down the Reichenbach falls with him.

Literature

Harry Potter has gillyweed.
Doc Savage not only had diving suits more advanced than anything available in Real Life at the time, but also 'oxygen tablets' that supplied oxygen directly to the blood and allowed the users to survive underwater for about two hours.

Live Action TV

Babylon 5: Several races need to use breathers when going into the alien sector and several alien races need to use breathers to travel the rest of the station. Also, G'Kar has gills surgically added that allow him to breath in alien atmospheres without a device. Lyta gets a set after being Touched by Vorlons.

Real Life

Underwater gear generally falls into two categories, tanks of compressed air, and rebreathers. The former exhales directly into the water (leading to the characteristic bubbles), while the latter simply filters out the CO2 and adds oxygen so the nitrogen can be re-used. It still has some gas, so it's only a partial example.
Mosquito larvae have a biological snorkel that they use to breathe from just below the surface of the water.
- All aquatic snails also have snorkels.

Tabletop Games

Early editions of Dungeons & Dragons
- A number of magical items had this ability, such as the Helm of Underwater Action, the Cloak of the Manta Ray, Potions of Water Breathing and the Necklace of Adaptation.
- Spells: the Airy Water and Water Breathing spells.
- "Natural" magic items: module EX1 Dungeonland had seaweed that allowed breathing underwater. There were similar items in other adventures.
- The most common/popular item throughout the series is the ring of waterbreathing.
The artificial gill and the gill suit from GURPS: Ultra-Tech allow people to spend hours or even days underwater. In High-Tech SCUBA and rebreather gear is available. Even as far back in time as Low-Tech it's possible to get waterproofed animal skins that give a few extra breaths if used correctly.
Dark Heresy has "Redole Rebreather", designed by Magos Genetus Halix Redole as he was working on colonization of a water planet. It's a helmet containing cloned piscine gill tissue. It isn't cheap and needs to be kept wet constantly when it's not used, but it never runs out of air and the fleshy part lasts a decade or so.

Toys

Bionicle: the Kaukau and Kaukau Nuva
- There are also some devices that allow water-breathers to survive above surface. These devices were somewhat significant in the 2007 storyline and onwards.

Video Games

Mario gets one for the Literal Bottle levels in Super Mario Sunshine. It extends the time he can breathe underwater without grabbing coins.
Zelda series has many items that serve this function:
- Zora's Flippers appear in many of the 2D games, and allows Link to breath underwater.
- The Legend of Zelda Ocarina of Time has the Zora Tunic that lets Link breath underwater.
- Oracle Of Ages has the Mermaid suit.
  - Both Ages and Seasons also have the Zora Ring, which allows you to stay submerged in shallow water for as long as you want.
- Twilight Princess has the Zora Armor. When Link is underwater, a small mask covers his nose and mouth, letting him breathe underwater.
Commander Keen 4 has a set of scuba gear, but it has an unlimited air supply.
Oxyale in Final Fantasy I
- Final Fantasy VI had the diving helmet, which enables Sabin, Cyan, and Gau to travel the underwater Serpent Trench back to Narshe.
- The underwater materia in Final Fantasy VII
The Aqualung item in Alter AILA Genesis. You actually can't go underwater at all until you get it. Also lampshaded when it's obtained, as the character that normally explains these kinds of things says she has no idea how it works.
Variation; it's a spell in MARDEK that can only be cast by a specific party member. A straighter example would be the Oxyale, a consumable potion that bestows the same effect.
Sonic 3 and Knuckles featured the bubble shield, which bestowed Super Not-Drowning Skills.
- Knuckles picks up the Air Necklace as one of his power-ups in Sonic Adventure 2, allowing him to stay under indefinitely.
In Cave Story, Curly Brace has an air tank... which makes the player character's susceptibility to drowning all the more frustrating. She eventually gives you the air tank.
Duke Nukem 3D has a scuba gear pickup which lets the player spend several minutes underwater, as opposed to maybe 30 seconds without it.
World of Warcraft makes extensive use of this trope:
- Class-specific spells; druid Aquatic Form, warlock Unending Breath, and shaman Water Breathing.
- Crafting provide water breathing potions and the Deepdive Helmet created by engineering.
- A weapon from Gnomeregan also functions as an Artificial Gill.
- As a whole, Blizzard learned during Wrath of the Lich King that underwater quests were a pain in the ass and most such quests now provide a renewable Artificial Gill buff. Cataclysm adds the game's first underwater zone, Vashj'ir, and the first couple of quests are to gain a permanent, zone-only Artificial Gill that also provides substantially faster swim speed, without which Vashj'ir would be all but unplayable.
In Mana-Khemia 2: Fall of Alchemy, the access to Deep Sea Ruins is opened after you synthesize Air Drop. Said item also returns in Atelier Rorona
Later games in The Elder Scrolls series have spells and enchantments for this available to players.
A signature item in the Build engine games. Duke Nukem 3D, Rendeck Rampage, and Blood all have it.
On the Online Games by Artix Entertainment, it is finally explained why you can breath underwater in all 5 games.
In Dragon Fable, you sink a ship full of water breathing potions that contaminates and enchants the ocean so that you can breath underwater.
Breath of Fire:
- Breath of Fire I required you to find a gill key item to let any character other than Gobi (a fishman) travel underwater.
- Breath of Fire II featured such an item as well.
Fallout: New Vegas gives you a rebreather if you help the Boomers, but other than a related mission and two caves there is absolutely nothing to see in the depths of Lake Mead. However there are certain areas of Vaults that happen to be submerged.
In the King's Quest II fan remake, a mermaid gives King Graham a kiss that somehow lets him breathe underwater.
Unreal has SCUBA gears available whenever the player must proceed underwater. They have a limited air supply that runs out pretty quickly. The expansion pack features a new one that not only lasts longer, but also recharges whenever surfaced.
- The SCUBA Gear reappears in the Oceanfloor assault map in Unreal Tournament.
Terraria has an accessory that allows for underwater breathing.
Wizard 101 has underwater breathing potions early in the game to allow access to a street that is completely underwater. However, due to the game mechanics, players can teleport to this area without adverse effects.
- Later there is a device in Celestia that allows the player to breath in the underwater areas. It was designed by the Celestians to survive when the Storm Titan betrayed them and flooded their world. Too bad they finished it too late.
Deus Ex has rebreathers that allow you to breath underwater for a limited time. The prequel features an Implanted Rebreather that does the same for poisonous gases but not water (or at least we don't see it do so, because you never go underwater).
In Guild Wars 2 every character has one standard, because the dev team did not want to limit the underwater areas' size to how long you could hold your breath.

Western Animation

Bart Simpson once used Milhouse's inhaler as a breathing device.
Samurai Jack: Jack is given a device to help him breathe underwater while swimming to the submerged Humongous Mecha.
The animated Flash Gordon had an episode with featuring a literal Artificial Gill: It was a metal collar with gills, that also prevented the wearer from breathing outside of water.
Kim Possible uses a rebreather provided by Wade in an episode where Drakken seals her in a filled-with-water bottomless pit.
Futurama: The crew is given rebreathers when they find the lost city of Atlanta.
The Gills of Hamachi from Xiaolin Showdown, although it achieves this by turning the user into a literal fish person.