In waterworlds where people live mostly under water or have access to bodies of water how is violence perpetuated in close quarters?
Projectiles lose their power really fast making them unviable. Bladed weapons are also slow. Explosives tend to hurt person deploying them as well. Grappling could be an option, but people tend to want an advantage, not equal fight.
How do you (gun)fight underwater?
In reality, hand-held harpoon guns used by divers generally use elastic cords to propel harpoons over fairly short distances to spear fish, a bit like an underwater crossbow. However, some firearms designed for underwater use are basically miniature harpoon guns, throwing slim darts — normal guns simply won't work for anything further than point-blank range underwater, if at all. (Not that darts carrying much further, but they can be optimised for the job.)
And here you have a list of real life underwater firearms, such as the Heckler & Koch P11.
Close quarters violence underwater wouldn't be impacted as much as you might think. There are four areas I can think of where they would be adjusted. In general, they are effected by two factors: water resists movement, and shorter range means it's harder to put an AoE attack far enough that it doesn't hurt you.
Heavy swung weapons like hammers and halberds wouldn't be effective because they would lose too much energy. Blunt trauma weapons like clubs or morning stars wouldn't exist.
The most efficient melee weapons would be spears and knives. You could do some sword-like things, but they would be mostly ineffective against armor. Aerodynamics would be a huge factor, changing the shape of anything swung to have the lowest amount of drag, and it would take more skill to hold the weapon at the right angle.
It occurs to me that you could have a axe-like weapon with a thin profile, but for which the wielder purposely turned the weapon in mid-flight to change its trajectory. Again, you'd want the Myth Busters to actually test the effectiveness of such a thing, and it's doubtful that it would be more effective than a spear.
This is the land of spear guns. There are numerous implementations of pistols and rifles that can get significant range under water. By "significant" I mean "pathetic by air standards." The equivalent of a sniper rifle has an accurate range out to around 60 meters, and your typical rifle will have half that.
The trick is that you use long, pointy projectiles. Darts, really. And then put a larger explosive charge behind it. You can extend the range by giving the bullet a flat or concave nose, which creates a tube of vacuum, eliminating drag along the edges of the bullet. Even then, a bullet will continue moving until it displaces its mass x velocity in water, which makes it a mass-to-cross section formula. By putting more mass behind a smaller cross section, you get significant range improvements.
The underwater equivalent to a gyro-jet wouldn't have adequate accuracy, as small water currents would throw it off. It would be like trying for a long golf putt.
Theoretically, you could have a gun that sets off a shaped charge, concussing everything in a cone in front of it. I don't think I'd be willing to test such a weapon, though.
The concussive force of an explosive is, in fact, conveyed more efficiently by water. It's almost impossible to throw a normal grenade far enough to not injure yourself.
Thus, we don't use normal grenades. Instead, we use tiny torpedos. Throwing one of these things like a javelin activates its propeller to send it swimming forth towards the enemy. When the propeller runs out, it explodes.
This is an area where the gyro-jet weapon could be useful. Basically, a tiny rocket-propelled grenade launcher. I'm sure this could be scaled up for anti-armor activities.
The problem with a super-cavitation weapon like the pistol shrimp uses is that, for less energy, you can fire a projectile. The shrimp only does that because it can't grow its own bullets. This could be the basis of a stunning melee weapon, where you poke it at an enemy and pull a trigger, releasing an elastically retracted clapper.
Flame throwers are a non-starter, but you could try sodium/phosphorus rounds.
Gas attacks, like tear gas (tear fluid, in this case), would have effectiveness based on the wet-suit tech used by the combatants. You could launch serious skin irritants into the water, but the shorter range would mean that it would be harder to limit the area of effect to just your enemies.
Acoustic grenades would come into their own. Something that vibrated the water in such a way that it irritated ear drums in the vicinity would be pretty easy to engineer.
Explosives tend to hurt person deploying them as well
only if there is nothing containing and directing the blast. Look at our world: an exploding grenade hurts everybody around it, however whoever uses a rifle has an explosion happening next to their cheek without getting hurt.
If you can direct the pressure wave, you can hurt without getting hurt. It's the same principle of lithotripsy
In extracorporeal shock wave lithotripsy (ESWL), external shockwaves are focused at the stone to pulverize it. [...] Electrohydraulic lithotripsy is an industrial technique for fragmenting rocks by using electrodes to create shockwaves. It was applied to bile duct stones in 1975. It can damage tissue and is mostly used in biliary tract specialty centers. Pneumatic mechanical devices have been used with endoscopes, commonly for large and hard stones.
Use animals
I don't know the tameability/trainability of the local aquafauna of your world/s but you could use marine life to deal damage to your enemy. Even if you only have access to relatively mindless/voracious predators you can feed them just enough to keep them alive/fit but not enough to sate their hunger, leading to them seeking out and attacking other living things once released.
Of course, you'll need to take measures to make sure they don't go after you as well, but the use of animals for warfare is not an entirely new concept as history teaches us.
(Solid Rocket Booster Bullet)
Basically, the gun doesn’t provide the propulsion, the bullet does. All the “gun” does is trigger the Solid Rocket Booster to fire. Then the SRBB has a burn time of X (insert time here based on caliber of “bullet” for your world) before it runs out of solid fuel and de-accelerates the ammunition.
Why not use the water itself as a weapon, like the pistol shrimp, which claw snap can create a shockwave that can stun or kill it's prey/ennemy.
Just have weapons along the line of a torpedo.
Since it's in water a good idea would be to have delayed explosion on a torpedo like missile. Shoot it in their vicinity and either blow it up remotely or have it on a timer.
