Fill Canteen in Cubes
At least the ones where it makes sense

Most cubes have a faucet in some form.

I'd suggest that you should be able to fill a canteen using that faucet.

It probably falls under the same reason that cubes require money to shower.

Without going into IC info, there are plenty of ways and places you can fill your canteen (some that even require no monetary input). So while I wouldn't be upset if this was added to cubes, I can understand that if it falls under the same reason cubes have pay showers, no hairstyler, etc.