I filled a Metal Tank with water to haul back to my base, and to my surprise I did not immediately become Overburdened. Indeed the filled tank (holding over 50L of water) weighed only 3.08 kg. Given that the empty tank weighs 2.83 kg, that means the water is about 0.5% as dense as it should be.
Edit: Semi related bug. When I attempted to use the water purifier in my Foodco Kitchen Buddy to purify the tank of water it told me that it ‘didn’t have enough charges’. The vehicle is running off a 99% full storage battery. I have to pour the water into a plastic jerry can, then purify that.