Regardless of trait, all ranged attacks have their range halved when made by or when targeting a creature that is underwater. All attacks that deal Bludgeoning or Slashing take a -2 penalty if they pass through water at all.
As with how being underwater affects weapons like say...a Skyfire Sword or Flame Thrower, you guess is as good as any as as per the example of Player Core 429, a Flaming Fangblade wouldn't be able to deal the Fire damage due to the Fire Trait of the module...a trait weapons with F damage or Flame weapon Group don't actually have AFAIK.
Normally I hear people ruling that fire damage is prevented as well. In addition to actions and spells with the fire trait. Because why wouldn't fire have the fire trait?
So while you could use a laser pistol underwater, it wouldn't do any damage.
But mostly you will want to work it out with the GM on what weapons do and don't work.
Maybe a laser pistol should work.
Maybe a zero pistol shouldn't.
Maybe a semiauto pistol shouldn't work unless it has a modification that lets it shoot its bullets underwater.
Honestly for a normal laser it probably makes sense for it to not work under water. Now there should be mods you can get that change the frequency on it so it can function underwater.
Also things like sonic weapons shouldn't work in a vacuum.
I suspect we should get a bunch more weapon mods with the tech core.
Also things like sonic weapons shouldn't work in a vacuum.
The alternative vacuum rules in GM core are more extreme.
Energy Damage in a Vacuum
In a vacuum like space, there’s no medium for energy transfer through conduction or convection. Cold, fire, and electricity spells or actions with the cold, fire, and electricity traits don’t function in a vacuum. Ignore these rules if the spell or action targets an adjacent target, even if it uses a ranged attack roll. This rule can be especially oppressive and should be avoided if the PCs are reliant on these abilities, and you should feel free to exempt specific traits. Magical effects or weapons with magical upgrades bypass this effect—magic breaks normal physics!
Of course there are a few problems with this.
(1) Weapon strikes aren't "actions with the cold, fire, and electricity traits." Monster ranged and melee attacks with elemental damage pick up the trait, but there's no rule applying that to PC weapons that do that via base damage type.
(2) Many fire actions/spells are going to be laser based, and aren't going to care about a vacuum. C'mon, guys.
(3) It's self contradictory about spells: second sentence says magical energy spells don't function, last sentence says magical effects do function.
Yeah those rules are a bit odd.
For cold weapons it is generally shooting some fluid or cold ice projectile at a target. It is transmitting the energy once it is in contact with the target. Now if you had some kind of pure cold non physical gun sure maybe that would not work.
Fire weapons have a few issues. For things like plasma guns it should have no impact on them look at the sun plasma works fine in space. For flame thrower weapons if the chemical they are shooting has its own oxidizer those should work fine in space until it burns its fuel out. For lasers it is a beam of light the effect on the target is heat/fire but that is not radiating through space it is from direct interaction with a physical medium of the target.
Electrical kinda depends how the weapon works but with space suites and their insulation and the vacuum arguing that those don't work or work well.