As is states I can’t work out how to fill up empty plastic bottles or canteens with water. I can fill them with gasoline from gas pumps just can’t work out water.
'e’xamine the water source and it should prompt you for a container to place water in.
If you want to transfer water from one container to another, 'w’ield the container with water, and then 'U’nload and again it should prompt you for a water container.
Thanks mate 
Note: Water sources include rivers, toilets, swamps (not recommended) and anywhere with water tiles (LMoE shelters for example) but not sinks.
Also you can move liquids from one container to another in your inventory. Choose the bottle to move liquids [U]from[/U] and then hit (U)nload. Not sure if it asks if you want to pour it on the floor before or after showing your inventory. Either way decline it (if needed) and select the target bottle. Liquids that cant fit (Canteen to bottle for example) will leave the rest in the first container. Canteen 6 -> water bottle = Canteen 4 and water bottle 2.
Which means you can reenact the water puzzle from Die Hard 3. 
Since when did C-4 or Mininukes have pressure-sensitive detonators?