Can you fill them from plastic water bottles?

Yes. (U)nload them. You’ll be asked to choose a container to pour the water into. Choose one of your plastic bottles.

You can use that system to interchange any kind of liquids between any kind of container you have in your inventory (from gallon jugs to canteens, canteens to plastic jerrycans, jerrycans to plastic bottles, etc).

If decide you don’t want to transfer the liquid or choose the wrong container then (ESC)cape the inventory screen when prompted, and when asked if you want to pour the liquid on the ground say (N)o.

You have to wield the container in your hands first IIRC, Crow.

That’s not actually needed anymore. You can access it from the inventory, select the canteen (or whatever container) and just press U. The same applies to the rest of actions. You’ll notice that by selecting something in the inventory it’ll show the list of actions available (although that must be something from 0.4 pre, anyway, even it doesn’t show you should be able to perform the actions).

Yeah, that was tweaked a bit in 0.4.

Behavior in 0.3: it shows a list of actions, with no regard to whether they’re possible or not.
Behavior in 0.4: it shows a list of actions, colored according to whether they’re possible. Impossible actions (unloading an apple, for instance) will still show, but should be visibly greyed out. Also, "D"isassemble has been added to the list.

I may have missed a few corner cases, but the coloring should be generally reliable.

Holy crap, that will save me a lot of time, thanks :U.