I've played mods before in which you can fill your canteen at sinks etc.
I bought a canteen of purified water and it says the canteen is empty. How do I fill it? Cheers.

Well, there's your problem.

Sussed it. Seems I need the 'Drink to last' perk.
Can't see why it wouldn't be possible to fill it without the perk. But there you go.