3. You have to manufacture ammo whereas the laser cannons recharge on their own.
More importantly, you have to manufacture ammo one shot at a time - it would be much less of an issue if it was "Gauss ammo (x50)"
I suspect that "second Gauss ammo" in the data was supposed to be something along those lines...
The very idea of using Gauss weapons underwater is unholy. It's like using torches underwater.
Gauss bullets are ultra-fast and quite small, which gives them limited range on land because the quickly burn in the atmosphere. Of course, water is way worse. So I don't think they could have made a worse choice for a weapon type than this.
Why am I writing all this? Well, if I were in charge, I would come up with some other weapon type and depending on choice, I'd give it logical stats. For Gauss cannon as it is, sure, we can give it a better range or something to increase its gameplay value, but that's still unsatisfying.
Strictly speaking "Gauss" only tells you what is used to accelerate the projectile - it usually comes up in the context of "how to build a gun with a higher velocity than gunpowder weapons", but there's nothing that'd prevent one from building a magnetic accelerator launching heavy, slow projectiles.
Arguably, such "slow" Gauss weapons are more suitable to underwater use than traditional firearms, because they don't care about external pressure.