Water pumps seem to be pretty common and I've found a few ponds but unfortunately even after taking the huge risk of dropping all my gear next to the pond and swimming around with only my canteen, I somehow never seem to get the option to fill the canteen, so I don't think I've figured it out... may have to try it out in the singleplayer mod to test, as swimming around naked while my Alice Pack and Winchester and stuff is in a convenient pile on the shore freaks me out, no easy fight or flight if a bandit shows up.
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