It seems like the water weight gain system is just a static multiplier on total weight. My plate carrier (contents: glock with ammo in holster, 3 grenades, various other stuff filling the other slots) got drenched in rain, and its weight in the details tooltip was 28kg. IIRC, without water, the weight is something like 14-16kg? The plate carrier is obviously not capable of retaining 12 liters of water; the max water content of it should be FAR lower, maybe 1-2 kg. 2x weight sounds about right for clothing, but for stuff like the plate carrier where most of the mass isn't absorbent, this is way off.