is there a way to be able to shoot while falling?
i have an idea for a little map where you fall forever(via teleport) and shoot people while falling.
semi off-topic: is it also posible to shoot underwater somehow

The answer to both is no. You can shoot while in a jet state above water, though, or you can shoot vehicle weapons underwater, but nothing else.

would it be possible to make a jet that makes you go down at a falling speed then

put this in your units odf
JetJump = "1.0" //The initial jump-push given when enabling the jet
JetPush = "0.0" //The constant push given while the jet is enabled (20 is gravity)
JetAcceleration = "3.0" // for characters with jet jump, use this acceleration for in air control
JetFuelRechargeRate = "0.0" //Additional fuel per second (fuel is 0 to 1)
JetFuelCost = "0.0" //Cost per second when hovering (only used for jet-hovers)(fuel is 0 to 1)
JetFuelInitialCost = "0.0" //4initial cost when jet jumping(fuel is 0 to 1)
JetFuelMinBorder = "0.0" //minimum fuel to perform a jet jump(fuel is 0 to 1)
JetShowHud = 0
JetEnergyDrain = 0.0
it is basicly a jetpack with no effect and only a tiny jump for a small bit of time but it allows you to fire after pressing jump twice and jumping off of somthing

whats weird though is, you can swing a lightsaber underwater. and i made a wrist flamethrower (really simple not worht anything) and on my jedi i could fire that underwater too.

Melee weapons are another exception. I don't know about your flamethrower.

If a unit's first weapon is a melee weapon, it can use its secondary attacks like force push and saber throw underwater.