I think using weapons while swimming or driving should be handled the same as dual wielding, except you’re only holding one weapon, it’s simpler that way.
We don’t know how ballistics will work in 4.X, but if drag (bullets slowing down as they travel) is a factor, and if damage is related to the speed of a bullet on impact, then bullets should suffer from extreme drag while in water. Weapons made for underwater combat needn’t have different ballistics above or below water, just have low enough drag that even while in water, they would remain effective.
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