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.