The Hero or the Big Bad, they can still fire completely accurately, despite the fact that Guns Do Not Work That Way.
In Real Life, this trope can vary. Most guns will jam or misfire when people discharge them underwater. It's not the gun itself that is affected (well, not for the first time, anyway) — it's the ammunition itself. Even if the gun is somewhat water-resistant (and that's a big somewhat, given that the barrel of a gun is essentially a giant hole), the primer of the ammunition is not. Besides the possibility of the primer being waterlogged, the gun's inner workings could also end up corroded by the water, and attempting to fire a corroded gun is a Very Bad Idea.
A gun may work one or two times when fired underwater, but in the long run, water will seep into the gun or wet the crucial parts of it, making it completely useless. And then there's the actual water pressure itself, which means the bullet won't go very far.
Also a case of Do Not Try This at Home, because guns are not intended to be fired underwater. Shooting a gun filled with water or wet ammunition is probably not going to end well for you or anyone around you. And shooting a gun underwater with air in its barrel is even worse, because the gun can explode.
Subtrope of Water Is Air, because the bullets will still work fine though they just won't travel very far. Spearguns designed for fishing underwater are also common in the hands of divers and underwater Mooks.
- In Black Lagoon, Revy gratefully takes an APS rifle from Balalaika, who used her military connections to the newly restructured Russian military to acquire it. She notably uses it during the Das Wieder Erstehen Des Adlers/Die Rückkehr des Adlers story when the APS is used against Aryan Socialist Union forces raiding a wrecked U-Boat.
- One of Leiji Matsumoto's short war stories depicts what happens when a rifle is waterlogged. A Japanese soldier, having been rescued from the sea, shoots the sole surviving crewman of a recently destroyed American submarine that sank his transport ship. The Japanese rifle, having gotten seawater down the barrel, manages to spit out the water along with the bullet. The bullet is slowed down by the water and only gives the intended victim a nasty bump on the head. To the credit of the Type 99 short rifle in question, the barrel's chrome-lined bore doesn't suffer any corrosion.
- John Wick: Chapter 3 Parabellum: John Wick and a Mook fall into a pool during the battle of the Continetal. The mook tries to shoot Wick, but the bullets do not reach him and he shoots the mook point blank instead.
- Jurassic World: Fallen Kingdom: Claire and Franklin are trapped in a leaking "hamster ball" under water. Owen fires his gun at the ball to help them escape, and it actually works until he gets hit by a blob of lava and drops his gun.
- Lethal Weapon 4: Martin Riggs kills Wah Sing Ku by shooting him point blank with an assault rifle after throwing him into the water.
- The climactic fight of Mindhunters downplays this: the heroine and the Serial Killer Big Bad both pack Glocks and both shoot at each other while struggling underwater, but while the Glocks shoot all right, the bullets just lose all of their velocity the moment they leave the barrel. The two of them are then forced to keep the guns out of the water and the fight then becomes a test to see who can hold their breath the longest, with the stakes of the loser being shot the moment they have to emerge.
- U.S. Marshals: While a pistol is not actually fired underwater, Sam Gerald makes mention of Glocks being capable of doing this as one of the reasons he loves his sidearm.
- Tested by the MythBusters. Even if a gun does fire, the water will slow a bullet down to nonlethal speed in short order.
- Alter A.I.L.A.: In the Underwater Facility, you can fight enemies while underwater - and the battle screen doesn't reflect the battle being under water. This means that you can use all the regular weapons that you'd be able to use normally note
- Call of Duty: Ghosts: This is a Justified Trope. As for the underwater missions "Atlas Falls" and "Into the Deep" Logan starts of with the APS Underwater Rifle for obvious reasons.
- Delta Force's standard guns won't fire underwater, which is a nice touch considering how little time players will actually spend swimming. The second game introduces a pistol and an assault rifle specifically for underwear combat, but they're notably inferior to standard guns.
- Guns won't work underwater in Deus Ex, so you'll want a melee weapon for dealing with critters in the sewers.
- Half-Life: One of the advantages of the Glock (the game's starting handgun) is the capacity to fire underwater. It is the only other weapon (the crowbar being the other) that can be used in such scenarios.
- In the Water Levels from Jazz Jackrabbit (Lagunicus in the first game and Marinated Rabbit in the second), all of the weapons successfully work underwater, including the fire bullets, without getting slowed down.
- Player Unknowns Battle Grounds: The player can swim through the water and their guns can fire as though nothing happened.
- The Legend of Zelda: Spirit Tracks: The train's weapon is a large cannon, but deals with underwater segments by firing torpedoes instead.
- Super Robot Wars: Most of the mechs' projectile weapons work underwater regardless of how these actually function in the canons of the works that are crossing over. Gameplay and Story Segregation applies, of course, but sometimes these can also be handwaved by the Minovsky Physics or Magitek involved with a particular series. Subverted because, depending on the installment, there might be some penalties to the damage output and/or accuracy of these weapons based on their terrain ratings. These ratings can be improved through upgrades, ace bonuses, or items, if these features are present.
- Team Fortress 2: Many of the weapons will work underwater, except for Pyro's flamethrower, and the flare gun.
- Terraria: Multiple types of guns can fire underwater with no difficulties. Flare Guns can fire underwater fine, as can the minishark, which fires musket balls.
- Justified in X-COM: Terror from the Deep. Since most of the fighting is done underwater, the X-COM soldiers use weapons specially designed to be used underwater. Some weapons only work underwater (for example, the torpedo launcher), while others work both under and above water.
- Pathfinder: Firearms can't be used underwater unless protected with specific magic, and even then, shooting through water imposes a stiff penalty on the weapon's accuracy. Early ammunition is ruined outright by exposure to water.
- Shadowrun: Averted, as there are rules for how bullets work underwater (generally, they don't) and there are a few weapons that have been specifically designed for underwater combat, including a carbine.
- Played with by the Soviet APS underwater rifle, which is Exactly What It Says on the Tin: An assault rifle for combat underwater, firing steel darts that resemble those fired by a Spear Gun. It was originally designed for the use by divers guarding important underwater infrastructure, after NATO successfully tapped underwater telephone cables carrying military comms and prompted the Soviets to start having their own frogmen carry out patrols. It was also issued to Spetznas for their own combat diver teams, but they quickly discovered it suffered from Crippling Overspecialisation: Out of the water its range and accuracy were poor. The ASM-DT amphibious rifle, which could fire the same darts as the APS but also load standard 5.45x39mm bullets, was developed to meet demand for a dual-purpose weapon.
- Some people just had to try firing ship-board artillery underwater.
- There was a news item making the rounds in 2016 about a Florida fisherman (of course) who used a Glock underwater to fish. Of note for the purposes of this trope: While the gun could fire multiple shots after some slight modification (apparently, all that needs to happen is the firing pin needs to be cut to allow to allow water to flow behind it and the springs are probably going to need replacing a lot sooner), he had to use a special (underwater only) sound suppressor, as the sound waves became sufficient to impact the shooter underwater, particularly with repeated shots.