While in the dambog bunker, between tier2 and tier32, I thought hey, I have a couple of water purification tablets, why not top up on the water stats.
I've tried using a canteen, a glass bottle and a jerrycan, but there's no option to fill them with water, no matter where you stand, and point.
Obviously doesn't work when swimming, but I'm talking about standing at the edge of the water, and it's not like it was too far away either; I tried it while the water was rising (using the red lever).